Python 3 compatibility

Hi,

I applied 2to3 in order to create python3 compatibility. Travis seems to think it works :) see the [build result](https://travis-ci.org/frensjan/django-multidb-router) for 2.6, 2.7, 3.2, 3.3 and 3.4. Confirmed in a local setup.

I'm not sure what to do with the unicode prefix in the docs. I guess sphynx is okay with the project and copyright being binary strings instead of unicode.

Best regards,
Frens Jan