'Unregisters the given model(s).
If a model isn\'t already registered, this will raise NotRegistered.'

def unregister(self, *model_list):

    for model in model_list:
        if (model not in self.registry):
            raise NotRegistered((u'The    model    %s    is    not    registered' % model.__name__))
        del self.registry[model]
