Fading menues and optional protocol for overriding gesture

Hi 
I added 2 features which I found useful in my project.

1. Added an optional protocol with 1 method which any UIViewController can choose to implement. Whenever user performs a slide gesture PKRevealController.m will now check if the frontViewController conforms to that protocol and if so let that controller to override the gesture. This is useful is several scenarios: eg if a ViewController for example has a UITableView and wants the slide-to-delete to work as expected (ie not sliding to left/right menu).

2. Added a property in order to make the menues fade in/out while sliding.