Added option to show mentions

Added support for [GET statuses/mentions_timeline](https://dev.twitter.com/docs/api/1.1/get/statuses/mentions_timeline). The user can select, in the widget and shortcode,  if he wants to display mentions or not (default = false). Mentions are merged into [user_timeline](https://dev.twitter.com/docs/api/1.1/get/statuses/user_timeline) and the merged timeline is sorted on created_at