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.
Nice work! For consistency, could you add a `var ` to the beginning of this line?

@jasoncalabrese (and other Pebble users) This code looks solid to me! And is a wonderful addition. But since I don't have a Pebble to test the code myself, I'm going to leave it to someone else to merge this code.