Rename methods in ViewController to be more predictable.

Change -keyboardWasShown to -keyboardWillShow and
-keyboardWillBeHidden to -keyboardWillHide.

The new names parallel the notification names.
Add some support methods to SGBubbleData.

Also remove unneeded synthesizes.
Simplify ContentCell and TypingCell with Left and Right subclasses.
Push the simplification a bit further.