[google|dns] Add Record resource

- Add Record models, requests and tests
- Add #record method (and associated test) to Zone
[google|dns] Add Change resource

- Add Change models, requests and tests
- Add #changes method (and associated test) to Zone
- Add #destroy method (and associated test) to Zone
- Add #modify method (and associated test) to Zone
- Add #save method (and associated test) to Zone
[google|dns] Add examples