code: function sendText (recipient, text) {
    return new Promise (async (resolve, reject) => {
      if (!text) {
        reject('text required');
      }
      
      let payload = {'text': text};
  
      try {
        let response = await this.callSendApi(recipient, payload);  
        resolve(response);
      } catch (e) {
        reject(e);
      }
    });
  }
docs: Sends a text message via the [Send API](https://developers.facebook.com/docs/messenger-platform/reference/send-api).
  @param  {Object}  recipient  An object that describes the message recipient in the format: `{<id_type>: <id>}`.
  For example, sends to a PSID would be `{'id': 123456}`, to a phone number `{'phone_number': '+1 (408) 444-4444'}.
  @param  {String}  text  The text to send.
  @return {Promise<Object>}  The API response
  @memberof  Client#
  @example
  let recipient = {'id': '57024957309673'},
  text = 'This is a text message';
  Client.sendText(text)
  .then(res => {
  console.log(res);
  // {
  //   "recipient_id": "1008372609250235",
  //   "message_id": "mid.1456970487936:c34767dfe57ee6e339"
  // }
  });