Adds isNodeAttached helper method.

PR 1 for `onAttach`

This adds a helper method `isNodeAttached`. It lets you know if an element is a child of the document or not. I also updated `triggerDOMRefresh` to take advantage of the new helper.
Mmm...I disagree. For one, we don't stub any of the other jQuery methods we rely on all throughout Marionette, of which there are many.

Further, there's no other way to test this other than literally rewriting the jQuery method itself, which we don't do because that's the whole reason that we're using jQuery. Right?