feat: battery percentage changed event (#126)

New "change_battery" event is triggered when the battery percentage changes. Has current battery percentage and charging state as object param.

Co-authored-by: Pedro S. Lopez <pedroslopez@me.com>

example.js

@@ -237,6 +237,12 @@ client.on('group_update', (notification) => {
     console.log('update', notification);
 });
 
+client.on('change_battery', (batteryInfo) => {
+    // Battery percentage for attached device has changed
+    const { battery, plugged } = batteryInfo;
+    console.log(`Battery: ${battery}% - Charging? ${plugged}`);
+});
+
 client.on('disconnected', (reason) => {
     console.log('Client was logged out', reason);
 });