Add Heart Rate Service sample for bluetoothLowEnergy

New sample app for the chrome.bluetoothLowEnergy API.
A lot of variables and functions polluting the global namespace. Can you use something like the [Module](http://addyosmani.com/resources/essentialjsdesignpatterns/book/#modulepatternjavascript) design pattern to avoid this? 

Ideally, if you have time, you could move the bluetooth handling into a reusable Object (using methods in the prototype) and keeping the DOM handling separate. See how I did this in the Google I/O Codelab: the sample-specific [main.js](https://github.com/mangini/cl/blob/master/main.js) and the reusable [board.js](https://github.com/mangini/cl/blob/master/board.js)