mention tagging. support February 6, 2013 changes to Facebook API

see #363

Add support for [Facebook mention tagging](https://developers.facebook.com/docs/technical-guides/opengraph/mention-tagging/) inside the post to Facebook Timeline user message.

![mentions-typeahead](https://f.cloud.github.com/assets/1100/113184/b8b8cb82-6b41-11e2-8645-d855680da2bb.png)

remove post meta boxes used to mention Facebook friends and pages in a post. remove front-end display of mentioned Facebook friends and pages.

Previous API methods allowing the ability to post to the walls of friends and pages is scheduled to be removed by Facebook as part of its [February 6, 2013 breaking changes]
(https://developers.facebook.com/roadmap/#february-2013).