Add transitionToParent() method

[gsklee]

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().

[nateabele]

I don't think we want to bloat the API with a bunch of case-specific transition methods. This is already being worked on as part of $state.go() (#15), which will allow relative transitions to parents, grandparents, children, siblings, etc.

Also, this method doesn't handle parameters at all, whereas $state.go() will correctly handle cascading parameter inheritance.

[gsklee]

Oh, I dunno there's such a plan, and it indeed looks better.