Make getOriginalValue null safe.

For new objects getOrginalValue depends on $changedProperties which may not have been set yet.
Merge pull request #24 from beckje01/nullSafeGetOriginalValue

Null safe get original value
Updates for non "id" identifier

Remove direct dependency on UUID as the id and allow custom id mappings
Merge pull request #25 from beckje01/cassandraIdMapping

Cassandra id mapping