Add transitionToParent() method

I added a `transitionToParent()` method since I found this being a must-have method when doing nested states and views. Was doing some manual splitting and joining with `$state.current.name` before discovering `$state.$current.parent.self.name`, so I believe this is worth a public API and some documentation.

Actually, I think the name of the method is too verbose; I name it this way to make it looks like existing methods. It would be great if `transitionTo()` can simply be `to()` so this method can be called `toParent()`.