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 wonder if it would be simpler/more correct to implement this using `takeUntil` passing in an observable that emits when the window is detached. It would be less code and would rely more on battle-tested core RxJava operators.