py3k compatibility via six

It see you've already made a few adjustments for py3k since the last release.  I've made a few more changes to get the tests all passing for python **2.6**, **2.7**, **3.2** and **3.3**.  

The change requires the inclusion of the `six` library, which does mean a dependency.  I think there was even prior discussion of using six, but it was discounted because of the metaclass magic factory-boy does - but it does look to be working.  The changes, as you'll see, are not many and do not require any re-org of the files.

I'm not sure if the travis build of this pull request will be calculated with the updated travis.yml (which has py3.2 and 3.3 in it), so here is the travis build which passed, just in case:
https://travis-ci.org/nkryptic/factory_boy/builds/4947134

I didn't change `setup.py` to include `six` in `install_requires` at this point.  I simply added it to the travis.yml file, so the tests would have it.  If this pull request is the direction you want to go with, I'll be happy to update it and include it.

By the way, awesome work on this project. And the changes to the docs, which are in progress, are looking good.