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().
`View.OnAttachStateChangeListener` here is an implementation detail. I would move it out the way `OperatorTextViewInput` does it.