authenticate(accessToken) {
    return this.request('AUTHENTICATE', { access_token: accessToken })
      .then(({ application, user }) => {
        this.accessToken = accessToken;
        this.application = application;
        this.user = user;
        this.emit('ready');
        return this;
      });
  }
Authenticate
@param {string} accessToken access token
@returns {Promise}
@private