Use KaTeX inside of perseus exercises

Summary:
Replace the <MJ> component with a <TeX> component that tries to render
math using KaTeX first, and then falls back to using MathJax if it fails. The
only big change is the way that that component is rendered, everything else
seems to work fine.

Test Plan:
Type some math that KaTeX can render. Add some math that KaTeX can't
render. Remove the extra math. Make sure the math looks sane throughout.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3121