Add AndroidObservable#bindView()

bindView() is a sister of bindActivity() and bindFragment().
Thanks for View.OnAttachStateChangeListener, it doesn't require
any manual unsubscription. It frees developers from overriding
View#onDetachFromWindow().
It's unfortunate that we have to expose all of these both in the public API and as non-`final` classes. I don't see why they should be. Again, this is something we can leave for now and address later.