DATACMNS-479 - Make sure Map types are not added to the mapping context.

Changed the algorithm for nested PersistentEntity detection to only use PersistentProperty.getPersistentEntityTypes(…) and make sure we return unwrap Maps and Collections inside the method correctly.
DATACMNS-478 - Allow build to be run on Java 8.

Introduced XmlUnit to make sure we can test XML rendering Java 6, 7 and 8.