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 think ``onready`` only happens when persona starts up though, right? Does it ever happen after a call to ``nav.id.request()`` ?