Set heat mode (#13)

* Function to set the heater mode + readme updates

* making the test set the heater to 'heat pump'

messages/SLSetHeatMode.js

@@ -0,0 +1,29 @@
+'use strict';
+
+const SLMessage = require('./SLMessage.js').SLMessage;
+
+const MSG_ID = 12538;
+
+exports.SLSetHeatMode = class SLSetHeatMode extends SLMessage {
+  constructor(controllerIndex, bodyType, heatMode) {
+    super(0, MSG_ID);
+
+    this.controllerIndex = controllerIndex;
+    this.bodyType = bodyType;
+    this.heatMode = heatMode;
+    // heatmodes:
+    // 0: "Off", 1: "Solar", 2 : "Solar Preferred", 3 : "Heat Pump", 4: "Don't Change"
+  }
+
+  encode() {
+    this.writeInt32LE(this.controllerIndex || 0);
+    this.writeInt32LE(this.bodyType || 0);
+    this.writeInt32LE(this.heatMode || 0);
+
+    super.encode();
+  }
+
+  static getResponseId() {
+    return MSG_ID + 1;
+  }
+};