Add message extraction from django templates
Fix support of template filters
Rename UPDATE_TRANSLATIONS_APPS to UPDATE_TRANSLATIONS_PACKAGES

This means translation files in a django project can also be updated
instead of being limited to apps.
Improve documentation
Add locales exclusion list for update operation.

This allows not having an empty translation file for the locale in which
translation message are written in the code.
Bump to version 0.4.0