Resolve conflict between side-menu pan gesture and ios-7 swipe-back
gesture.
Added a new configuration "disablePanGestureWhenSwipeBackEnabled" to

The problem: In a view with system "Back" button, the user can go back
by simply swiping from the edge. The SideMenu's pan gesture overrides it.

Fix:
Now pan gesture is automatically disabled if the following satisfies
1. disablePanGestureWhenSwipeBackEnabled == YES, and
2. The content view controller is a UINavigationViewController, and
3. There are more than 1 view controllers in the navigation controller's
stack, and
4. interactivePopGestureRecognizer.enabled == YES
Then the user can trigger the swiping back gesture correctly.