Added support for remote + passworded access

This password encoder was decompiled from the Android app then manually cleaned up and ported to Javascript. It ain't pretty, but it works.

I don't know that this is all necessarily the most correct or idiomatic way to implement these features, but I wanted to make sure the functionality got committed to help out people wanting to use this for smart home appliances (#1).

messages/SLChallengeMessage.js

@@ -5,8 +5,21 @@ const SLMessage = require('./SLMessage.js').SLMessage;
 const MSG_ID = 14;
 
 exports.SLChallengeMessage = class SLChallengeMessage extends SLMessage {
-  constructor() {
+  constructor(buf) {
     super(0, MSG_ID);
+
+    if (buf) {
+      this._wroteSize = true;
+      this.writeBuffer(buf, 0);
+
+      this.decode();
+    }
+  }
+
+  decode() {
+    super.decode();
+
+    this.challengeString = this.readSLString();
   }
 
   static getResponseId() {