getMembership(username, cb) {
      log(`Getting membership of user ${username} in Team ${this.__teamId}`);
      return this._request('GET', `/teams/${this.__teamId}/memberships/${username}`, undefined, cb);
   }
Get Team membership status for a user
@see https://developer.github.com/v3/orgs/teams/#get-team-membership
@param {string} username - can be one of: `all`, `maintainer`, or `member`
@param {Requestable.callback} [cb] - will receive the membership status of a user
@return {Promise} - the promise for the http request