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 either of these `@param`s are useful, but there are similar ones in this file already. Leave them for now and we'll purge all of the useless ones in a later pull request.