Add Set<NotificationState> to NotificationEntry to track things like New, Complete, and Favorited

https://issues.jasig.org/browse/NOTIFPLT-47

This changeset introduces the concept of states to Notification objects and refactors the FAVORITE feature to become a state.
See later question about whether state is historic or current only.  I'm thinking this should be current state only, and in future another method would provide historic state auditing.