Interactive2 $(document) calls now unbind specific handlers

Summary:
$(document).unbind calls are now careful! This commit includes a khan-exercises substate
that implements the same behavior in Interactive1.

Test Plan:
- Run Perseus.
- Add some $(document) handlers that log.
- Move around some Interactive2 items and notice that your $(document) handlers persist.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D10751