Added trivial messageID generator on client side

parser/src/main/java/org/dna/mqtt/commons/MessageIDGenerator.java

@@ -0,0 +1,20 @@
+package org.dna.mqtt.commons;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ *
+ * @author andrea
+ */
+public class MessageIDGenerator {
+    private AtomicInteger m_current;
+    
+    public MessageIDGenerator() {
+        //NB we start at -1 because the first incAndGet has to return 0
+        m_current = new AtomicInteger(-1);
+    }
+    
+    public int next() {
+        return m_current.incrementAndGet();
+    }
+}