Add square roots (\sqrt)

Summary:
Follow the TeXbook instructions on how to construct square roots. Using
makeCustomSizedDelim, this becomes nearly trivial.

Test Plan:
 - Make sure normal tests work
 - Make sure the new huxley test looks good, and other huxley tests haven't changed.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12918