Backporting expressions/equations

Summary:
Added "expression" as a new answer type in answer-types.js
Will copy over inline comments to public wiki as well.

Test Plan:
Switch "Factoring Polynomials 1" to "expression" answer type and verify
that it both works as expected and matches functionality present in Perseus.

Reviewers: alpert, eater

CC:

Differential Revision: http://phabricator.khanacademy.org/D3257