Add in popViewControllerAnimated subclass method.

Add in tests for navigation controller back patch. on_back method now added for screens. Implemented a functional test to ensure that this works, but still lacking some unit specs. 

I think a new pull request is in order for some more coverage on the navigation module and some further tests for this (though I'm unsure where to place those unit tests). Let me know your thoughts on this commit and where / how I can update wiki/readme.
Hm, I see a problem here. This will only work for the root view controller. So, if you're 5 view controllers deep and "pop" back, the `on_back` will be fired on the root view controller instead of view controller #4.