isCollaborator(username, cb) {
      return this._request('GET', `/repos/${this.__fullname}/collaborators/${username}`, null, cb);
   }
Check if a user is a collaborator on the repository
@see https://developer.github.com/v3/repos/collaborators/#check-if-a-user-is-a-collaborator
@param {string} username - the user to check
@param {Requestable.callback} cb - will receive true if the user is a collaborator and false if they are not
@return {Promise} - the promise for the http request {Boolean} [description]