pidstore: model relationship
pidstore: model relationship

* Adds SQLAlchemy model relationship to allow easy access to
  log message for a given persistent identifier.

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

tiborsimko/invenio-pidstore,inveniosoftware/invenio-pidstore,tiborsimko/invenio-pidstore,inveniosoftware/invenio-pidstore