Basic equation support. Comparison and form checking works.

Summary:
Simplification still has a few kinks (e.g. 2x=2y doesn't get simplified).
Refactored most of the logic out of compare.js to keep things DRY.

Test Plan:
src/expressiontools/test.html
Using equations on my local copy.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2672