Make docker-py compatible with python 3

I've been playing with docker-py in a project of mine that is running under python 3.3 and I got a couple of exceptions. This PR adds ``six`` to make the code compatible with python 3.3. Since there were only a few places that needed changing I thought about not using ``six`` but working aroudn it. Especially for ``iteritems`` in line 76 this seemed quiet impractical and I think ``six`` is the right fit. Let me know if you think this is sensible or needs to be changed.