Refs #13 - Introduces common django config then overrides it for all exi...

...sting django versions available today. No magic for now, new configs classes will be needed when releases will come.
In https://docs.djangoproject.com/en/1.5/releases/1.5.3/

> Django 1.5.3 introduces a new setting, SESSION_SERIALIZER

Please handle this change, or do not support versions lower than 1.5.3.
Notice that it means that SESSION_SERIALIZER must **not** be part of the base settings.