Added support for Python 3.

This pull request adds support for Python 3.
The changes are fairly minor and fix the Python 3 issue mentioned at Bitbucket: https://bitbucket.org/carljm/django-lockdown/issue/8/python-3-support
I haven't tested all features of django-lockdown with Python 3, but ```2to3``` doesn't show additional changes and for me it works quite well with Python 3.