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().
`bindActivity` and `bindFragment` should be `{@link #bindActivity}` and `{@link #bindFragment}` for convenience.