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
Is `#be_all` just using the regular RSpec magic to call `#all?`, or is there a specific matcher?

If there isn't a specific matcher, the output from the second approach is almost certainly going to be clearer if this spec fails.