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().
I don't think `assert` is the right thing to do here. Android doesn't even run these. Shouldn't these be a normal `if` conditional?