Bug 892523 - [User Story] Email Invocation from Notification

Switches over to using the new Notification approach now that [bug 910915](https://bugzilla.mozilla.org/show_bug.cgi?id=910915) has been fixed. Also needed to add an onclick handler since the message handler is not fired if the app is running. Once this lands, it will address the "email notification grouping" story too.

It also looks like our message handler is fired twice in some cases by the system, so a "uid" is added to the notification data so that we do not duplicate actions. I will file a bug for the system notification issue, but this allows us to land now, and may even be useful in the future.