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().
@hamidp Thanks for the suggestion!
Apparently takeUntil() doesn't unsubscribe() the observer, so I don't think is is desirable in this specific case.