BibClassify: API string support

* Add new API function bibclassify_exhaustive_call_text to run
  BibClassify on a string.

* Performs module-wide PEP8 and PEP257 updates.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Guillaume Lastecoueres <PX9e@gmx.fr>