code: function postback(payload, callback)
  {
    if (typeof payload.payload == 'string')
    {
      payload.payload = JSON.parse(payload.payload);
    }
  
    callback(null, payload);
  }
docs: Parses provided payload
  Note: always expects postback payload to be JSON
  
  @this  Fbbot#
  @param {object} payload - messaging envelop object
  @param {function} callback - invoked after parsing is done