Refactor all policies.

We now have three categories of policies:
1. Password strength policies;
2. Password change enforcement policies;
3. Authentication policies.

Each policies now inherits from one of the three base policies.
Every category of policies has its own handler to execute all policies
chosen by the Django project.

The settings have been refactored completely. Please look at the testsite
settings for an example on how to use Django Auth Policy.