Sync with Khan-exercises

Summary:
Add some things in to make this more useful to khan-exercises.
Notably, make KaTeX.process create and clean up its own .katex node, and rename
.mathmathmath -> .katex.

Test Plan: Make test, make sure the main page still renders stuff

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3061