Small commit to conform to a true container view controller

I love your custom navigation view controller, I have ton of ideas to make it better (navigation items support, navbar support, new API for customization...).
Look at https://github.com/Dimillian/DMGesturedNavigationController a cutom navigation controller of mine.

I moved you custom view controller implementation to a true container view controller.
Now correct methods are called on your child view controller, and the parentViewController property will not be nil. 
So I also modified your category, you don't need to add a runtime property anymore. Just to cast the parentViewController methods. 

Hope you like it ! I'll add more modification later, on my fork.