Implement swipe-to-reveal delete button in the thread cells

Some more work on making the thread view controller more like iMessages.

- Removes the avatar image view on the thread cells
- Adds SWTableViewCell as a pod for an easy way to implement swipe-to-reveal delete button on table view cells
- Revealing the delete button on cells also animates the bar button items and adds an overlay view on the searchBar

P.S. There are still some bugs with SWTableViewCell that need to be fixed, mainly related to the SWTableViewCellDelegate method
``` - (void)swipeableTableViewCell:(SWTableViewCell *)cell scrollingToState:(SWCellState)state```

![http://i39.tinypic.com/3130f9i.gif](http://i39.tinypic.com/3130f9i.gif)