CRM-14516 allow filtering and clean up media_buttons function

Switch function from media_buttons_context to media_buttons.  Used priority 100 to keep button as the 'right most' button to account for other plugins adding their own buttons.  

Incorporates the changes from PR #38 (https://github.com/civicrm/civicrm-wordpress/pull/38) related to civicrm.php.