Add a .create action to RestfulModel

* Create a `FailedRequestError`
* Models now include `.create`, `#save!`, and `#persisted?`
* Add a shared example for creatable models
* Add a create action to `ResourceGateway`

Should we do something to handle errors here? The JSON parsing could fail, or the expected key could be missing, in which case a `NoMethodError` for `first` on `NilClass` probably wouldn't communicate the failure very well.