Add bitly integration to dropplets

$short_url can be used in a theme in place of $post_link. It will automatically convert the $post_link to a bitly link if the bitly username and bitly api key are entered correctly in the settings. Otherwise it falls back to $post_link (or results in an incorrect information failure that is evident from the shared text) I have changed it in the "Share on Twitter" and "Comment on Twitter" in the default theme as an example.