BibCheck: new creation_date check

* Exploits the BibCheck infrastructure to backdate the
  creation date of a record based on the actual content of
  its metadata. In this way tools such as BibSort will be
  able to properly sort records according to their real
  creation.
  (closes #1719)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>