BACKGROUND 1. Field of Invention This invention relates to disposable coupon dispensers suitable for outdoor use. 2. Cross-reference to Related Applications This application claims the benefit of U.S. Provisional Application No. 60/624,214, filed November 2, 2004, and claims the benefit of U.S. Provisional Application No. 60/586,377, filed July 8, 2004. 3. Incorporation by Reference The disclosures of United States Patent Nos. 5,944,219; 6,053,356; 6,267,263; and 6,364,099, are hereby incorporated by reference herein. 4. Prior Art and Other Considerations The patents incorporated by reference above are directed to disposable coupon dispensers which include a dispenser box that houses a stack of coupons in the form of small sheets of paper or paper-like material. Above noted patents, nos. 5,944,219; 6,267,263; and 6,267,263, show coupon stacks in which each coupon in a stack has a portion coated with a lightly aggressive adhesive that removably connects it to an immediately adjacent coupon in the stack. The dispenser box has a dispensing opening or slot in one wall thereof. The stack of coupons is biased toward the dispensing slot by a spring housed between the opposite wall of the box and the stack of coupons. The adhesively-coated portions of the coupons are so located on the coupons and the coupons arranged in an accordion fashion that, as one coupon is removed through the dispenser slot, the next adjacent coupon in the stack is pulled partly through the dispensing slot so that it can readily be clasped and pulled out of the dispenser box. The coupon dispenser assemblies also include mounting members which enable the coupon dispenser to be mounted on retail store shelves, wire racks, cardboard displays, or in other locations convenient to customers. Above-noted patent no. 6,364,099 recognizes a need for coupon dispensers which can be used to dispense coupons outdoors, and provides a reusable protective shroud to which a disposable coupon dispenser box can be removably mounted. Although the shroud provides a level of protection for the disposable dispenser, the coupon at the top or outer end of the stack and extending partly out of the dispenser box is subject to damage and deterioration caused by rain or other precipitation. Coupons in the box can also get wet and deteriorate from water entering through the dispensing slot. Coupons in present use comprise papers that could not properly function in a dispenser exposed to various outdoor weather conditions, particularly moisture caused by outdoor weather conditions including dew, fog, and precipitation such as rain, snow and sleet. To function in any dispenser from which the coupons are manually pulled out, a coupon must remain intact when pulled out of the coupon dispenser. Otherwise, the coupon could tear apart when pulled out of the dispenser. In addition, the adhesively-coated surface of the coupon must be capable of retaining an adhesive coating after exposure to adverse weather conditions. Otherwise, the removal of one coupon would not necessarily cause the next coupon to be pulled out of the stack. Coupons made from papers used in disposable coupon dispensers in the past have been made from paper which is not water-resistant. Such paper is referred to herein as "typical paper" in contrast to water-resistant paper used in accordance with the present invention. SUMMARY This invention provides a disposable coupon dispenser that is readily utilized in outdoor locations in which there is little or no protection from weather elements such as dew, fog and rain, snow, or other forms of precipitation. In accordance with this invention, such a dispenser is provided by including coupons made from water- resistant sheet material instead of typical paper coupons used in the prior art disposable coupon dispensers. Materials which are flexible, tough, durable, highly resistant to damage from exposure to water and various climatic conditions, are readily available and useful in the practice of this invention. Moreover, techniques are available for printing long-lasting images with striking colors on some types of water-resistant paper or plastic sheets. Sheet materials that could be used in the practice of this invention may comprise specially coated papers, such as papers having clay-based coatings (referred to herein as "clay-coated paper") available, for example, from Precision Press, Inc. of North Mankato, Minnesota, and many polymers including, for example, polyesters, polyolefins, such as polyethylene and polypropylene, and polyvinyls, which are widely available. In every case, the coupons must be made from a sheet material which is water-resistant. As used herein, a paper is considered to be water-resistant if coupons made therefrom are capable of remaining sufficiently intact when manually removed from a dispenser to pull another coupon partly out of the dispenser despite exposure of the dispenser to days or weeks of outdoor weather conditions, including dew, fog and rain or other form of precipitation. Many plastic sheets could be considered so water-resistant that they can be considered to be waterproof. However, such high degree of water resistance, while useful in the practice of this invention, is not necessary for the practice of this invention. A coupon dispenser in accordance with this invention comprises a dispenser assembly comprising a container having a hollow interior and a fastener connected to the container for mounting the container on a support. The container houses a stack of water-resistant coupons within its hollow interior and the container has a dispensing slot though which the coupons may be removed by manually pulling the coupons from the stack outwardly through the slot. The dispensing assembly is constructed to prevent water from reaching the hollow interior of the container except through the dispensing slot . Other features and advantages will become apparent from the following description and claims and from the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a coupon dispenser assembly which may be used in the practice of this invention and illustrating the adhesive connection between one coupon and a second, next adjacent coupon. FIG. 2 is a view similar to FIG. 1, but shows the adhesive connection between the second coupon of FIG. 1 and a third coupon next adjacent thereto. FIG. 3 is an exploded isometric view of the coupon dispenser assembly of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawings, a coupon dispenser assembly 10 that may be used in the practice of this invention comprises a flexible support sheet or placard 12 made from a waterproof plastic material, such as a polyvinyl, having an adhesively-coated back surface 14 covered for shipping and storage by removable protective sheets or release liners 16. The adhesive used to coat the back surface 14 may be a mildly aggressive, repositionable adhesive which enables the placard 12, following removal of the protective sheets 16, to be mounted on and afterward removed from a mounting surface without leaving an adhesive residue on the mounting surface. Advertising messages can be printed on the front surface 18 of the placard 12. The coupon dispenser assembly 10 also includes a transparent, plastic, box-like coupon dispenser housing or container 20 that houses a stack 22 of water-resistant paper or plastic coupons 24 which, as typical of merchandiser's coupons, may provide benefits such as discounts to holders of the coupons. The container 20 has an open back surrounded by a flange 23 adhered to the front surface ,18 of the placard 12 by an adhesive 29 or by being sonically or otherwise welded to the placard 12. In effect, the portion of the placard 12 behind the container 20 closes the otherwise open back of the container and serves as the back wall of the container 20. The construction of the dispenser assembly 10 renders the hollow interior of the container 20 waterproof except for a coupon dispensing slot 26 discussed below. As with coupons made from typical paper or paper-like material, the water-resistant coupons 24 may be adhered to one another by repositionable adhesive portions 25 along respective alternately opposite margins of the coupons 24. Compare FIGS. 1 and 2. The housing or container 20 has a dispensing opening or slot 26 in its front wall 27 through which coupons within the stack 22 are pulled one-at-a-time from the outer end of the stack 22 remaining within the housing 20 as a coupon 24 previously pulled from the housing 20 is pulled away from the stack 22 for use by a recipient. (As used in this specification including the claims, the "front wall" is that wall which has the dispensing slot 26. Also, the "outer end" of the stack 22 is the end of the stack 22 nearest the front wall 27 and the dispensing slot 26, and, similarly, the "outermost" coupon in the stack 22 is that coupon nearest the dispensing slot 26.) The length and width of the dispensing slot 26 are appropriate to enable the coupons 24 to be dispensed without jamming and will depend upon the size of the coupons 24 to be dispensed. Unnecessarily larger dispensing slots should be avoided to restrict the degree to which the coupons 24 in the container 20 are exposed to moisture. A spring 28, which can be made from suitable folded sheet material, such as card stock or a plastic such as polyvinyl, located between the placard 12 and the stack of coupons 24 biases the stack of coupons 24 toward the container front wall 27. Coupon dispenser assemblies similar to the assembly 10 but filled with typical paper coupons are available from Service Litho-Print, Inc. of Oshkosh, Wisconsin, and from Outta the Box Dispensers, LLC, of Dayton, Ohio, and from other sources. As an alternative to the repositionable adhesive found on conventional POST-IT brand notes and the like, the coupons 24 may be coated with a strip of non- repositionable adhesive that vanishes, fades, evaporates, or otherwise dissipates once a coupon 24 is removed from the stack. When the outer coupon 24 in a stack 22 of coupons is removed through the slot 26, the non-repositionable adhesive acts the same as a repositionable adhesive in the sense that the next coupon 24 in the stack 22 is pulled by the adhesive on the outermost coupon partly outwardly through the dispensing slot 26. In contrast to a repositionable adhesive, a non-repositionable adhesive is so dissipated upon removal from the dispenser 10 that it is incapable of adhering the removed coupon to another object. The details of such "one- shot" adhesives do not form a part of this invention and, therefore, are not further described herein. As further alternatives, coupons which have no adhesive layer could be stacked and folded together so as to be frictionally connected so that a first coupon pulled off the stack will pull the next coupon in the stack partly out of the dispensing slot while, at the same time, such first coupon separates from the next coupon. Various other coupon stacking arrangements which cause a coupon in a stack to be exposed for removal upon removal of a covering coupon have been suggested and may be usable in the practice of this invention. More generally, any coupon stacking arrangement which enables the manual removal of coupons one-at-a-time from a stack through the dispensing slot 26 may be used in the practice of this invention. Coupons comprising fifty-one pound clay-coated paper with a one-shot adhesive available from the aforementioned Precision Press, Inc. of North Mankato, Minnesota, has been used in the coupon dispensers of this invention. However, the water-resistant paper or plastic coupons 24 in accordance with this invention could have any thickness or weight appropriate to the uses to which the coupons are intended. In general, the coupons 24 could have characteristics similar to the typical paper coupons used in prior art disposable coupon dispensers. However, the water- resistant coupons 24 of this invention are made from materials, such as those mentioned above, which provide the distinct advantage of substantial resistance to deterioration due to exposure to outdoor weather conditions including dew, fog and rain or other forms of precipitation. Although a particular form of coupon dispenser container and mounting member therefor are illustrated and described herein, it will be appreciated that the particular form of container or housing and mounting member therefor can be varied to suit the needs of the user or manufacturer thereof. For example, the housings and mounts therefor could take any form shown in the prior patents mentioned above and incorporated by reference herein, provided that their housings are modified to be essentially waterproof except for the presence of coupon dispensing slots.