Added a more useful toString() method.

src/main/java/net/floodlightcontroller/core/types/MacVlanPair.java

@@ -44,4 +44,8 @@ public class MacVlanPair {
     public int hashCode() {
         return mac.hashCode() ^ vlan.hashCode();
     }
-}
\ No newline at end of file
+
+    public String toString() {
+        return "(" + mac.toString() + ", " + vlan.toString() + ")";
+    }
+}