Start implementation of Guzzle 4.x

This serves as a starting point for Guzzle 4.x where the namespaces have changed. I haven't been able to get this to work yet, it seems like the data sent to the token url isn't correct (probably due to changed calls).

Details on migrating from 3.x to 4.0 are here:
https://github.com/guzzle/guzzle/blob/master/UPGRADING.md#3x-to-40