Fixes #873

Note: This solution does incur a small performance penalty in that each
callback first checks if the view is still open. Only one other unit test was
affected by the change, because it used a callback which executed after close
of view, and if I understood the other unit test correctly, it remained solved.
A couple other options would be rewriting several things in Backbone (unbind
algorithm, event trigger method, etc), or writing a more specific override
which only kicks in if the callback method has some special property. This
includes a test case.

Squashed commit of the following:

commit 46f7c581a974fb9746941ec6d4242d33fa31e117
Author: Jason Schmidt <Zannalov@GMail.com>
Date:   Sat Jan 18 17:22:41 2014 -0500

commit 3d292d815fe6d5c38c38a71b0b2798d3c0bb81e0
Author: Jason Schmidt <Zannalov@GMail.com>
Date:   Sat Jan 18 16:31:27 2014 -0500