Removed debug logging

Added verification of broadcast response message size
Added clearer handling of message types

I'm sure there's a better way to do debug logging, but I don't really feel like adding a dependency on another module just for that.

messages/SLChallengeMessage.js

@@ -1,7 +1,13 @@
 const SLMessage = require('./SLMessage.js').SLMessage;
 
+const MSG_ID = 14;
+
 exports.SLChallengeMessage = class SLChallengeMessage extends SLMessage {
   constructor() {
-    super(0, 14);
+    super(0, MSG_ID);
+  }
+
+  static getResponseId() {
+    return MSG_ID + 1;
   }
 }