NEW
- `core.models`: BasePHistorical, BaseHistorical, NullOwned, Owned, SoftDeleted
- `core.tests`: create_user(username, password, **kwargs), get_user(username)
- `core.shortcuts`: get_object_or_none(model, **search_keys), validate_and_create_object(model, **kwargs), validate_and_update_object(model, search_keys=None, **kwargs), update_or_create_object(models, search_keys=None, **kwargs)