Bump Django from 1.5 to 1.6
Make tests discoverable by Django 1.6's new test runner
Explicitly set default value for all BooleanFields

In Django 1.6 they default to None instead of False.
django.core.xheaders was removed in Django 1.6
commit_on_success() was deprecated in Django 1.6 and replaced with atomic()
Switch back to using pickle to serialize sessions