added french SIRET and SIREN numbers
added models for SIREN and SIRET fields
added prep_values() for SIREN/SIRET, according to common display format
metadata commit : updated authors, changelog and "versionadded" tag on relevant
classes.
Merge branch 'master' into french-sirene

Conflicts:
	AUTHORS.rst
Minor updates to new SIRENE fields to handle the abstract base class. Other cosmetic changes.