Enhancement: Added GitHub Repository Service

Added GithubService and moved controller methods to service
i will remove this method because this is only used in https://github.com/zendframework/modules.zendframework.com/blob/master/module/ZfModule/src/ZfModule/Controller/IndexController.php#L70 - i wrote with @localheinz that maybe the cache implementation can be tossed as the github client allready caches api calls. suggestions?