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'