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().
Yeah I agree. It's the way it's done in core so we followed suit. One solution could be to move `AndroidObservable` to the operators package as a public factory class and make operator impls package local. Same for `ViewObservable` etc.