Allow model_to_dict to hydrate multiple paths

[cgearing]

At the moment, model_to_dict can take a path of relationships to hydrate, for example:

model_to_dict(person, ['parent', 'sister']) would result in hydrating the parent relationship on the Person model, and then hydrate the sister relationship on the Parent model.

It would be nice to be able to hydrate multiple relationship paths with a single call, an example could look like:

model_to_dict(person, ['parent', ['sister', 'brother]]