A few tweaks to move cornice closer to Python 3 compatibility.

These are a few tweaks towards making Cornice friendlier to Python 3. The tests are still not passing -- there is more work to do, but I wanted to share what I have thus far in case I'm heading down the wrong path. For example I added a dependency on [six](http://packages.python.org/six/). Hopefully, this is OK.

Tox results:

```
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py32: commands failed
ERROR:   py33: commands failed
```