Best Sellers Page for Zencart 1.3+

This mod will create a best-sellers page that can be accessed at ?main_page=best_sellers
Unlike the "best sellers reloaded" mod, this one does not have its own template, it uses the product_listing template (the template used for listing category products)
The reason for that:
1. It keeps the mod extremely light weight.
2. Keep the listing consistent with the product_listing.