add saucelabs cross-browser testing

@yields @ianstormtaylor @calvinfo thoughts on how we'd test saucelabs from travis?

- https://github.com/defunctzombie/zuul
- https://docs.saucelabs.com/reference/sauce-connect/
- http://docs.travis-ci.com/user/sauce-connect/
- https://github.com/admc/wd
- https://github.com/visionmedia/mocha-cloud