Added stackable mock adapter + docs + tests

With guzzle3 we were able to add responses in a stack, and let those be returned respectively. That currently only seems to work with the integrated node.js solution - but that didn't work for me, so I decided to create a stackable mock adapter.

It takes arrays / responses and you can also add them using the addReponse method. (similar to the node.js solution)

I'm not sure if this follows the way things go around here, so let me know if I'm wrong here.