Add a predicate answer type

Summary: I really hope this doesn't break _All The Things_

Test Plan:
Create a test exercise with a solution like this:

    <div class="solution" data-type="predicate" data-forms="integer">function(n) { return n === 5; }</div>

Test locally, verifying the correct answer is accepted and incorrect answers aren't. Verify the timeline works correctly.

Also check an existing exercise, degrees_to_radians, to verify number answers still work and work if they're part of multiples. Also verify the check answer message appears for pi approximations and verify that the timeline works.

Reviewers: alpert, cbhl

Reviewed By: alpert

CC: Dave

Differential Revision: http://phabricator.khanacademy.org/D2129