OGM-582 Reference Neo4j transaction in the 'Getting Started'
OGM-548 Additional javadoc and comments
OGM-592 Misc. improvements

* Improving method names
* Improving toString() method
* Protecting against null parameter
OGM-592 Using manual assigned ids for some test entities for the sake of simpler debugging
OGM-592 WIP Moving "collectionRole" from AssociationKey to AssociationKeyMetadata
OGM-592 WIP Moving "associationKind" from AssociationKey to AssociationKeyMetadata
OGM-592 WIP Removing obsolete logic from AssociationPersister