Bug 971450 - [System2] Instantiable Activities r=alive

https://bugzilla.mozilla.org/show_bug.cgi?id=971450

Screenshot of JSDoc:

![screen shot 2014-02-12 at 6 10 39 pm](https://f.cloud.github.com/assets/122602/2156127/1c623fa6-9454-11e3-9cdc-dc84a44a2c47.png)