Faraday 0.9 compatibility

Works with Faraday 0.9. Still works with Faraday 0.7–0.8 as well. Check the new build matrix, yo!

I had to resort to the awful monkeypatch to provide backwards compatibility with the `preserve_raw` setting. I will investigate whether there's a saner way.

Obsoletes #74. This PR prolly still needs some middleware registration tweaks, though.

/cc @pengwynn @atmos @sferik @gottfrois @technoweenie