bug fix to overlay code, comments and pragmas, demo app

This PR includes three main changes:

1. Comments and re-arrangement of code in RSAvatarController, to indicate which functions are their to conform to other delegates, and to guide users implementing the RSAvatarControllerDelegate

2. a bug small bug fix in RSAvatarController, to pull the overlay view over the scrollable image view, so that controls in the overlay will actually be accessible

3. A new class DemoViewController, implementing a basic demo where you can take a photo, move and scale it, accept the result, and see the result.