added french SIRET and SIREN numbers
added models for SIREN and SIRET fields
added prep_values() for SIREN/SIRET, according to common display format
Fixed a few Sphinx build errors.
Simplified promises Sphinx extension and added support for lazy dict values.

This also makes the lazy objects render as unicode strings on Python 2.
Replaced some calls to gettext with the lazy version.
Add Jinja2 to docs requirements to try to fix a RTD build error.
Added USZipCodeField model field. Fixes #55.

This also extends the documentation to make the difference to the US Postal Service model and form fields clearer.
Added authors file to documentation. Fixes #111.
Updated source translation catalog.
Updated translations.
Add checksums module from django.utils.checksums
Merge branch 'add_checksums'
Fix a few typos / spelling errors.
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.