Use post_class() in product loop

Use [`post_class()`](http://codex.wordpress.org/Function_Reference/post_class) in the Loop too instead of only on single product pages.

Update: I also added two extra classes for products that are featured or on sale. This helps styling a lot. Say you want to give products on sale a different background color, the `<span class="onsale">` alone in the "sale-flash.php" template is not going to help much since it is contained within the product `div` or `li`.