Bug 971479 - [System2] Instantiable CellBroadcastSystem

https://bugzilla.mozilla.org/show_bug.cgi?id=971479

Screenshot of jsdoc:

![screen shot 2014-09-15 at 11 15 33 pm](https://cloud.githubusercontent.com/assets/122602/4283419/7056e828-3d6e-11e4-908f-8f1769b3ce43.png)