Parse invocations of sqrt using {}

Summary:
MathQuill outputs sqrt expressions like \sqrt{2} etc, which are not currently
parsed. This adds the braces as a valid form of using the sqrt function.

Test Plan:
  * Open test.html
  * See tests 13 and 14 in the "square root" section pass

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D6092