Python3 support + django 1.7 in tests

This explicitly tags bytestrings, uses the built-in mocks, and initializes django settings for 1.7.

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. Breaks django versions prior to 1.5.

Manually tested set/get with Python 3.4 + django 1.7 (production environment).