BibField: initial release

* Introduces a new module 'BibField' that creates an abstraction
  for the concept of field, which permits (i) defining virtual
  fields (calculated or derived) and (ii) abstraction from MARC.
  The non-MARC master formats will come later.  (closes #1300)

* ISBN/ISSN checks were co-authored by Lars Holm Nielsen.

* Merge note: several minor kwalitee fixes.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>