Silver Challenge (Chapter 7) - Implemented pinch to zoom

Informed BNRAppDelegate to conform to the UIScrollViewDelegate protocol. Set the scroll view's delegate to the BNRAppDelegate. Removed local variable declaration of BNRHypnosis view and used an instance variable defined in the BNRAppDelegate's class extension. Set the scrollView's max zoom to 2.0. Implemented the scroll view delegate method viewForZoomingInScrollView method to return the instance of BNRHypnosisView.