Add an #update action to RestfulModel

* Models now include `#update`
* `RestfulModel#save!` uses `#update` if the object has been persisted
* Add a shared example for updateable models
* Add an update action to `ResourceGateway`
Yes, these keys are required for the update to succeed. I'd have thought that the `id` should be enough, but the API requires that the `email` (in this case) be provided as well. Any ideas on how I could name this better?

And sure, I'll add some documentation on the superclass method.