Add a Field model, refactor Client

* Add a `Field` model with an index action
* Rename `Client` --> `ResourceGateway`
* A `ResourceGateway` is associated with a particular resource and takes an `endpoint` and a `resource_name` as constructor arguments
That does read better. It's unlikely that we'd be returning a subclass of `Namely::Field`, and I'm not sure it should raise an error if we did.