Remove notification level after hiding. Fixes #200

@tilgovi this is a start on notifications work--submitted mainly to get feedback, doesn't seem mergeable yet.

I'm having trouble getting the notifications test to fail. First off, [the relevant assertion](https://github.com/okfn/annotator/blob/master/test/spec/notification_spec.coffee#L29-L30) is typoed, but if I change it to verify that the class `annotator-notice-show` is gone, the test passes. It shouldn't.

I do see in the dev.html page that, with this change added, the annotator-notice-* classes are removed on hide.

What am I missing?

I've got a start on queueing notifications, will code it up after I get the tests fixed. Basic idea is, show() queues up the message/status, then calls _show(). _show() checks if the queue has length; if it does, shift the zeroth entry and display it. hide() should call _show() after hiding the element, to get to the next item in the queue after the element is clicked or the hide timer runs out. Feedback welcome on this approach.