Treat any `<a class="facebook-button">...</a>` as a generic button.

This patch allows app developers to use HTML like `<a href="…" class="facebook-button">…</a>` to create a generic button. Previously, for button-like spacing, there needed to be a `<span>` that also had to have one of three classes that matched a sprite image. This is awkward in cases where the sprite images don't really match the button functionality. For instance, the HTML to implement a logout button might look like `<a href="<?php print $FB->getLogoutUrl();?>" class="facebook-button"><span>Log out of Facebook</span></a>`, but this would result in a "speech bubble" icon appearing next to the "log out" button. Dropping the `<span>` and applying the generic spacing CSS to the `facebook-button` class directly is clearer and more semantically appropriate.