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
I agree, that's a good idea, too. There's no reason to have these tests associated with profiles. I'll come up with some dummy data that we can use instead.