implemented NACK

Author: Michael Freund

lib/stomp.js

@@ -399,6 +399,19 @@ Stomp.prototype.ack = function(message_id) {
     this.log.debug('acknowledged message: ' + message_id);
 };
 
+
+//
+// ## Stomp.nack(message_id)
+//
+// **Deny received message**
+//
+// Takes a string representing the message id to nack
+//
+Stomp.prototype.nack = function(message_id) {
+    send_command(this, 'NACK', {'message-id': message_id});
+    this.log.debug('denied message: ' + message_id);
+};
+
 //
 // ## Stomp.begin()
 //