addTo method of attr.js should throw an error if it attempts to override an existing method or attribute in the class

[semmypurewal]

This isn't happening yet because of our approach to implementing inheritance and making the instanceof operator work. We set the prototype to an instance of the super-model but then override each individual attribute with an actual cloned attribute. This is necessary to maintain encapsulation of the attribute.

We'll have to re-think our approach slightly to make this work.