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().
nit: capitalize "Nothing". [Comments should be sentences][1].

 [1]: http://nedbatchelder.com/blog/201401/comments_should_be_sentences.html