code: function(conversationId, userId, text, callback) {
        send(null, conversationId, utils.messageText('user_id', userId, text), callback);
      }
docs: Send a plain text message from `userId`
  
  @param  {String}   conversationId Conversation ID
  @param  {String}   userId         User ID
  @param  {String}   text           Message text
  @param  {Function} callback       Callback function