added support for pan gesture forwarding

If the contentViewController is not a standalone controller (for example, it's a navigation controller), you might want to provide access to the menu on some child controllers but not others. All pan/swipe gestures, however, are currently handled by the menu controller. 

This change provides a protocol that interested content controllers can implement; If the pan gesture is disabled on the menu controller itself at the time of the gesture, this protocol allows the menu's contentViewController to handle the gesture differently.