Improve contributor guidelines

* Link to docs, JSON API standard, and thoughtbot's style guide
Add a CurrencyType model with an index action
Implement a Field model with an index action

* Add a resource_name property to models, since sometimes
  resource_name != models
Rename Client -> ResourceGateway

* ResourceGateway constructor takes an options hash
ResourceGateway take better constructor arguments

* Require `endpoint` and `resource_name` in the initialize hash
* ResourceGateway methods no longer require these as arguments
Rename site_name -> subdomain in configuration
Add a default resource_name to RestfulModel

* To be overwritten by subclasses if necessary
Unroll testing loop
Test ResourceGateway with WebMock, not VCR

* There's no reason to rely on profile data
Add a Country model with index and show actions
Extract RestfulModel tests into shared examples