Summary:
Add \displaystyle, \textstyle, \scriptstyle, and \scriptscriptstyle commands.
Added tests and huxley screenshots for everything that looks different in
displaystyle vs normal style.
Fixes#24.
Test Plan:
- See new tests work, and old tests still work
- See no huxley screenshots changed, and new screenshot looks good
Reviewers: alpert
Reviewed By: alpert
Differential Revision: http://phabricator.khanacademy.org/D13079
