Fix for issue #111 - iOS 5 compatibility

At the moment PKRevealController is unusable at iOS 5 if you don't use a **left AND a right** view controller. If you just use one (either left or right) shouldAutorotateToInterfaceOrientation will always return NO and this will let the application crash. (See my issue report https://github.com/pkluz/PKRevealController/issues/111)