dateutils: strftime improvement

* Adds a new function strftime to dateutils to use instead of
  Python stdlib datetime in order to support before 1900.
  (closes #1065)

* Adds tests for improved strftime function.

* Changes standard strftime function or adds exception handling
  in bibsort, bibauthorid, bibcirculation and websearch.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>