Added Relation::unlinkReference()
Added UnlinkReferenceTest
RecordCollections keeping references up-to-date through add() and remove()
Changed Relation for updating record references
Fixed Relation::setReferenceFor() when exchanging a collection for OneToMany-relations
little cleanup of Relation class
Implemented unit test updating relation references through RecordCollection::remove()
Implemented unit test for setting relation references
Fixed dataset cleanup for unit tests
Added unit test for testing the NULL-setting of owner field on OneToOne-relations
Added Connection::queryOne()
Added set of relation references for existing records (OneToOne and OneToMany relations)
Some cleanup
Added Relation::hasReferenceFor()
Refactoring Relation class (still in work)
Added ReferenceMap
Continued refactoring Relation/ReferenceMap classes (still in work)
Changed Relation
  renamed getReferencedRecord() to getRelatedRecord()
Continued refactoring Relation/ReferenceMap classes (almost done)
Continued refactoring Relation/ReferenceMap classes
- changed method visibilities to private
- added doc comments
Added feature to override query class in RecordManager
Changed Record
  added toArray() with recursive flag
  added fromArray() with recursive flag
Continued refactoring ReferenceMap class
Added method explanations in doc comments in ReferenceMap class
Fixed Relation::hasReferenceFor()
Fixed Record::toArray()
Fixed Query::getLimitedQuery(), wrong place for OFFSET