Increased support for TeX symbols returned by MathWidget

Summary:
On iOS, the MathWidget returns some symbols that KAS can't understand. I've added
support for a few of those symbols that might be relevant, namely the asterisk sign for
multiplication, the \neq sign for inequality, and the hyperbolic trig functions (these
probably aren't necessary, but weren't difficult to add). This doesn't cover _everything_
that MathWidget can return, but it probably covers everything we care about across the site.

Test Plan: See test.html.

Reviewers: alex, jack

Reviewed By: jack

Subscribers: jack

Differential Revision: http://phabricator.khanacademy.org/D11329