Added ability to set pool or spa heat setpoint

Fixes #9

messages/SLSetHeatSetPoint.js

@@ -0,0 +1,27 @@
+'use strict';
+
+const SLMessage = require('./SLMessage.js').SLMessage;
+
+const MSG_ID = 12528;
+
+exports.SLSetHeatSetPoint = class SLSetHeatSetPoint extends SLMessage {
+  constructor(controllerIndex, bodyType, temperature) {
+    super(0, MSG_ID);
+
+    this.controllerIndex = controllerIndex;
+    this.bodyType = bodyType;
+    this.temperature = temperature;
+  }
+
+  encode() {
+    this.writeInt32LE(this.controllerIndex || 0);
+    this.writeInt32LE(this.bodyType || 0);
+    this.writeInt32LE(this.temperature || 0);
+
+    super.encode();
+  }
+
+  static getResponseId() {
+    return MSG_ID + 1;
+  }
+};