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().
in general, `observeOn` should appear "on the right" in a sequence of steps, `subscribeOn` on "the left".

it wouldn't change semantics here, but I think it's better style to invoke `observeOn` last.