Add huxley screenshot tests

Summary:
Add in a testing page, which just renders the location hash in the body. Use
this to make some screenshot tests in huxley. Note, these screenshots were made
in Firefox on a Linux computer, so running the tests somewhere else might
produce something else.

Test Plan:
- Serve KaTeX (`make serve`)
- Run a Selenium Server (I used selenium 2.40.0)
- Run `huxley` from the test/huxley/ directory
- Ensure that all of the tests pass

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7258