Update pebble-js-app.js - Battery Status

Added code to the Pebble app that will allow you to view the current Android battery level of the CGM Uploader.  And display it on the Watchface App.
For users that don't have the uploader updated yet, this change would display `--%` instead of `undefined%`
~~~
currentBattery = (bgs[0].battery) ? bgs[0].battery : '--';
~~~