Django-dcat

- Fixing Agent type field. It is recommended and not mandatory.

- Add `Keyword` model
- Add `homepage` property to Catalog
- Make data-theme code unique and longer since it can be slug of label
- `license` property has been renamed to `licence`
- `themes` and `description` are now optional fields (Dataset)

**Full Changelog**: https://github.com/pdelboca/django-dcat/compare/v0.0.1...v0.0.2

Initial release