Added decoding of messages

Also moved things out to their own files and setup require()s to bring it all together.

messages/SLChallengeMessage.js

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