Document cross-browser & CI testing setup on college-choice

[shawnbot]

The setup is documented in this pull request, but it needs to be summed up and explained in plain English. Some things to touch on specifically:

• Configuring Travis to run tests against the live Jekyll server was tricky.
• Using Sauce Connect can produce false positives. At one point I wasn't even running the Jekyll server on Travis, but the tests were succeeding because I still had a tunnel open, and Sauce was just running the tests against the site running on my laptop.
• Explain the Nightwatch configuration, and how having the config parsed as JavaScript (rather than JSON) made things much easier to manage.
• Explain the multi-browser testing setup and the scripts that call Nightwatch once per browser.