Allow to pass class to `parse()`?

[SpaceK33z]

If you want to replace a model with another model, without instantiating a new class, you now need to do .parse(model.toJS()). Perhaps we can do this automatically. Or maybe this should be a separate function, which replaces that model with the new one while keeping the instance.

[SpaceK33z]

I'm leaning towards making this a separate function, because this can have some nasty side-effects. If the model was not instantiated from the same store, it could have different relations then the other models in the store.