Managers abstract methods should probably raise NotImplementedErrors

[timmartin19]

This would prevent dumb errors that I tend to make. Like calling the wrong super. E.g.

class MyManager(AlchemyManager):
     model = MyModel

     def create(self, values, *args, **kwargs):
          super(AlchemyManager, self).create(values, *args, **kwargs)
          # Do something else