constructor(auth, apiBase = 'https://api.github.com') {
      this.__apiBase = apiBase;
      this.__auth = auth || {};
   }
Create a new GitHub.
@param {Requestable.auth} [auth] - the credentials to authenticate to Github. If auth is
                                 not provided requests will be made unauthenticated
@param {string} [apiBase=https://api.github.com] - the base Github API URL