Add test for DataPreservingTransactionTestCaseMixin
Declare dependencies in setup.py
Fix data restore in Django 1.5/1.6 in DataPreservingTransactionTestCaseMixin

It was broken by Django 1.5 flushing data at a different time from Django 1.5 https://docs.djangoproject.com/en/1.5/releases/1.5/#database-flushing-in-django-test-transactiontestcase