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().
Hmm, there are SafeSubscriber that does unsubscribe the source. Probably that's fine to use takeUntil(), I slightly prefer the current way as it sems leaner as it doesn't introduce another Observable to talk from the listener to the returned observable. 

I don't have strong opinion here though.