NOTIFPLT-47:  Add NotificationState to NotifictionEntry to track things ...

...like New, Complete, and Favorited

https://issues.jasig.org/browse/NOTIFPLT-47

This PR is a re-boot of an earlier one and reflects the discussion there.
That is a great suggestion -- and I thought about it seriously.  The issue I have with using *that* representation of a timestamp is that the Notification API packages as a .jar that is brought in as a dependency by other projects.

Use of Joda Time sounds great... but I'd prefer to keep it internal.