Impl/ezp-22217 implement rest country service

The goal here is to implement a new REST ressource that can get a list of all the countries that are implemented in a YAML file ( fieldTypes.yml ). The ressource can be call with : GET /api/ezp/v2/services/countries .
The specs are from: https://gist.github.com/bdunogier/8665017
JIRA link: https://jira.ez.no/browse/EZP-22217

The unit tests are not done yet