Cross browser testing with Sauce Labs

Sauce Labs now provides free test time for open source projects, yay!

Test results can be viewed at:
https://saucelabs.com/u/s2js-probes

This commit provides a primative test runner that utilizes Travis CI to
drive Buster tests on Sauce for major browsers (and then some). While a
bit hacky in places, it's stable enough to start using. Converting into a
configuratble/resuable Grunt task would be ideal.