Resolve conflict between side-menu pan gesture and ios-7 swipe-back

Resolve conflict between side-menu pan gesture and ios-7 swipe-back gesture. 
Added a new configuration "disablePanGestureWhenSwipeBackEnabled".

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.

Also added sample code in the storyboard sample. 

![ios simulator screen shot dec 10 2013 5 32 50 pm](https://f.cloud.github.com/assets/746943/1720508/2ee348a0-6204-11e3-952a-790a892cdb88.png)

![ios simulator screen shot dec 10 2013 5 33 15 pm](https://f.cloud.github.com/assets/746943/1720510/3813e15a-6204-11e3-966b-df980c4a9c30.png)