Add style changing functions

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