Refactored to improve UX -- before, cells were choppy for a variety of reasons like 1.) not removing NSNotifications, 2.) NSDateFormatter (thanks Nick!), 3.) too many animations at once.

The refactor -- only 1 NSNotification is added now on JSMessagesViewController rather than in each JSBubbleMessageCell.  NSDateFormatters are for the most part static (except in the MHPrettyDate lib).  And it only animates when it moves back to the original position.  It is a lot smoother now.