Centralise persona callback state (bug 1048583)

* Fixed up stubby to have onready and call it to be closer to what persona does.
* Removed the centralised personaCalledBack state and got a test that failed.
* Re-added the centralisation changes and test passes.
* Tidied up some of the helper.js for consistency and to aid testing the attr change.
I suppose this should be an instance variable of SessionModel. It doesn't matter much because ``app.js`` only creates one instance but it would allow SessionModel to be unit tested.