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
Can you reorder the imports so it goes
```python
import itertools
import random
from distutils.version import LooseVersion

import django
from django.conf import settings

from .pinning import this_thread_is_pinned, db_write  # noqa
```

This just keeps it in three logical groups: stdlib, django, project.