Python 3 and Django 1.7 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
Please don't bump the version. We'll do that when it's release time.