Fix error reporting in CitySDK exporter.
Fix CitySDK exporter, correct id type.
Fix elasticsearch requirement name.
Freeze Django REST Framework version.

Update to latest version when either of
the following is fixed:

https://github.com/tomchristie/django-rest-framework/issues/1809
https://github.com/deschler/django-modeltranslation/issues/263
Add compatibility with REST Framework 2.4.

Circumvents https://github.com/tomchristie/django-rest-framework/issues/1809
Initial kulke keyword importer.
Import original kulke categories.

Allow kulke and yso categories to be used in filtering.