XDR handling separated from withCredentials, fixed #35

Take a look before I merge it.
There's no such thing as too much peer review.
let's get saucelabs set up with tests.

See:
 
 - https://github.com/defunctzombie/zuul/wiki/travis-ci
 - https://github.com/Raynos/mercury/commit/f151ba547bc1ea31e86420a17e1bca9f1fdfcd42