1st implementation for issue #7 (squash commit from issue#7)

DBRefs are working for most use cases. Autodeletes (unset,cascade) won't
work for now.

    - fix argument check (collections must equal not the reverse)
    - fix repository test not to report false negatives
    - Implemnt DBRef aware updateReferences
    - Get repository from set value
      Value might be inherited and have different repository.
    - Add tests for DBRefs with referencesOne
    - implement dbrefs for referencesOne