Added APParallaxViewDelegate protocol with optional methods APParallaxVi...

...ew:willChangeFrame: and APParallaxView:didChangeFrame:

Added APParallaxViewDelegate protocol with optional methods APParallaxView:willChangeFrame: and APParallaxView:didChangeFrame:

This removes the need of the user to create a custom observer to handle frame changes. 
e.g If the user uses a UICollectionView as a parallax view, he will most likely also need to invalidate it's layout before the frame changes.