Add the \KaTeX command

Summary:
Add a command to print out a TeX-style logo for KaTeX. I think the
code is okay, but suggestions for the design of the logo are most welcome (I'll
post screenshots soon).

Test Plan: Make sure \KaTeX renders, and the tests still succeed.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3443