code: function getNotification(account, identifier, urlBase, callback) {
    validate.address(account);
    validate.paymentIdentifier(identifier);
  
    return getNotificationHelper(this, account, identifier, urlBase, callback);
  }
docs: Get a notification corresponding to the specified
  account and transaction identifier. Uses the res.json
  method to send errors or a notification back to the client.
  
  @param {Remote} $.remote
  @param {/lib/db-interface} $.dbinterface
  @param {/lib/config-loader} $.config
  @param {RippleAddress} req.params.account
  @param {Hex-encoded String|ResourceId} req.params.identifier