Warn on unsimplified answer if it's required

Summary:
Should avoid future problems like this:
http://www.reddit.com/r/Khan/comments/1wedae/so_many_dang_glitches/

The issue was that the problem required simplification but the expected answer
was "0.01cos(2pix/1.2)", when the simplified answer was "0.01cos(5/3pix)"

Test Plan:
* Open test.html
* Require simplification and put in an unsimplified answer
* Should get a warning
* Try simplifying and turning off simplification and verify everything
  works as expected

Reviewers: alex, jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6381