Django 1.7 compatibility

The new app-loading feature needs that external scripts (like the test runner)
call `django.setup()` before doing anything else.

The new migrations feature needs a way to serialize (deconstruct) fields. Also,
it needs a way to deconstruct models.

The hack on the WorkflowEnabled class is needed, it seems, because of
http://stackoverflow.com/questions/11209521/base-metaclass-overriding-new-generates-classes-with-a-wrong-module.
It was trying to serialize it (in the migrations) as
`django.db.base.WorkflowEnabled`.