Update backbone.js

I am not sure if this breaks any principles in Backbone, but found it useful so I thought I'd share it.

Extended the toJSON method to handle referenced Backbone models as nested attributes to the top level attributes. Could include it as an argument, but it basically does what _.clone does.

When using Backbone models as a building block for views, not a syncing model part of a collection, you can find yourself wanting to reference a model within a model and use them both (all) in a template.

An example of this is when you have a model with its own view specific attributes and you want to reference the attributes of an other model.