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/SLGetGatewayDataMessage.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const SLMessage = require('./SLMessage.js').SLMessage;
+
+const MSG_ID = 18003;
+
+exports.SLGetGatewayDataMessage = class SLGetGatewayDataMessage extends SLMessage {
+  constructor(buf) {
+    super(0, MSG_ID);
+
+    if (typeof buf === 'string') {
+      this.writeSLString(buf);
+      this.writeSLString(buf);
+    } else if (buf) {
+      this._wroteSize = true;
+      this.writeBuffer(buf, 0);
+
+      this.decode();
+    }
+  }
+
+  decode() {
+    super.decode();
+
+    this.gatewayFound = this.readUInt8() !== 0;
+    this.licenseOK = this.readUInt8() !== 0;
+    this.ipAddr = this.readSLString();
+    this.port = this.readUInt16LE();
+    this.portOpen = this.readUInt8() !== 0;
+    this.relayOn = this.readUInt8() !== 0;
+  }
+
+  static getResponseId() {
+    return MSG_ID + 1;
+  }
+};