Saucelabs rewrite

Rewrote the integration with saucelabs, eventually extracting the logic to `co-webdriver-runner`

@juliangruber If you'd like to take a look at this (and co-webdriver-runner) that would be highly appreciated - I'm using quite a lot of your repos at `co-webdriver-runner` :)
i'd thunkify `server.listen` so you don't need the callback