toJSON include related models.

Template frameworks like Handlebars use the results of the `toJSON` for the data that is available to them to use. I've found it convenient to include related models in the `toJSON` response almost similar to the Eager Loading idea ORMs have. This is helpful when rendering say a post, and want to include the author or react based on the type of author for instance.

In the model we just set `withJSON` with the models to be included:

    var Post = Supermodel.Model.extend({
        urlRoot:App.apiURL+'posts',
        withJSON: ['user', 'image']
    });

The toJSON function will both test if the relationship type exists, and if this model has one related. If one exists- its toJSON is called and the result is assigned to the name of the relationship.

Their might be a better option name than withJSON. Feel free to test, I thought this might be nice.