Make sure that the menu view is always in front

Make sure that the menu view is always in front of other subviews of contentViewController
or else it may hide behind them. 
I experienced this problem while using a contentViewController which was not a UINavigation controller subclass and using the UIViewController containment APIs to add child viewcontrollers.
Merge pull request #28 from pujabelwal/master

Make sure that the menu view is always in front