BibField: Python-2.4 compatibility fix

* Fixes Python-2.4 compatibility problem.  (closes #1533)

* Also standardizes the use of double quotes arrounf XML atributes.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>