Python3 support

Add support for Python3 by being explicit about byte strings. Also, initialized settings in the django tests (which is required in at least django 1.7) -- based on @aroxby's fork. This uses a feature of django 1.5+ to convert back to strings as necessary, so this breaks support < 1.5.

Tests pass with Python 3.4 + django 1.7, Python 2.7 + django 1.7, Python 2.7 + django 1.6, Python 2.7 + django 1.5.

Manually tested set/get with Python 3.4 + django 1.7 (production environment).