Contextual Undo feature

This implements a ContextualUndoAdapter, used as a Decorator to add a GMail-like Swipe To Dismiss + Contextual Undo behaviour. To see it in action navigate to the ContextualUndoActivity in the example.

There's still lots of duplicated code in the OnTouchListener, but I wanted to keep the pull request small without changing too much existing code.
This should be fairly intuitive to use, even without commenting the code. If you prefer to have Javadoc comments on the main functionalities please let me know and I'll try to follow your style.