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
Actually, I'm unclear on the preference here. Do you like:

```ruby
expect(fields).to be_all { |field| field.is_a?(Namely::Field) }
```
or
```ruby
fields.each do |field|
  expect(field).to be_a Namely::Field
end
```
I remember some sound and fury about expectations in loops from a while ago, but I can't recall what we decided...