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.
Suggestions: Document what the `Date` means semantically in this `Map`.  Consider using [a better representation of a timestamp](http://joda-time.sourceforge.net/apidocs/org/joda/time/Instant.html) than a `java.util.Date`.