Implementation of #27 (mention friends):

	- Uses new check-in flow
	- Works for both check-ins and venue searches
	- Support for multi-user mentions
	- Compatible with existing shout editing functionality
	- Allows user to search for friends by voice
	- Shows an error message if a user's friend search returns no results

* Currently does not check-in mentioned friends, (just notifies them that they have been mentioned). This is believed to be a current limitation of the Foursquare API.