Add Created and Updated timestamps to JPA entities

Created is set when entity is Stored
Updated is set when entity is Changed

Dates used in REST response to represent Last-Modified.

New interface to represent the concept:
 getCreated
 getLastModified

LastModified is LastUpdated if it exists else Created

New BaseEntity handle @Id and Created/Updated dates via @PostCreate/@PostUpdate hooks in JPA. Similar behavior is needed for the non JPA domains, e.g. Attachment.

Issue: #41