Add a check to make sure ClusterableModel is used where needed

[johnraz]

This is just a proposal but I think it would be nice if we could the Django check framework in order to raise an error when a ParentalKey or a ParentalManyToManyField is used but the model does not inherit from ClusterableModel.

In my case I had to dig and guess on why my many to many field was not saved although everything was executing properly.

Just a suggestion - I don't know how hard that would be to implement.

[gasman]

This already exists for ParentalKey (#35) - sounds like we're missing the equivalent for ParentalManyToManyField.