Change khan-exercises to use BigBingo

Summary:
It's apparently hard to hook into our require system, so instead of using the
simple BigBingo client library, I'm making the ajax call directly.

Test Plan:
-Temporarily comment out the Khan.submitIssue lines in khan-exercise.js.
-Open the homepage and start a problem.
-Submit an issue and look at the requests in the mini profiler. The conversion
properly shows up.

Reviewers: eater

Reviewed By: eater

Subscribers: benkraft, mattfaus

Differential Revision: http://phabricator.khanacademy.org/D10442