Update for Backbone model for deep keypath

As I asked in https://github.com/RactiveJS/Ractive-adaptors-Backbone/issues/4, I don't really understand why Backbone models are only updated for immediate child property changes. I'd love to use deep models here.

This pull request fixes this behavior, but maybe there is a good reason for it. If so, I would happy to know why we should only propagate changes for immediate child property changes.