Import remaining metadata to CKAN

This adds logic to import most of the remaining data from ODP to a CKAN instance. 

Important missing data (which will likely have to be tackled with other tools):

- Unpublished datasets (because they're not available via the ODP API)
- Create / Modify dates (because they can't be modified in the CKAN API)
- User ownership (because user data isn't accessible via the ODP API)
- Data formats (because they are not tied to particular resources in ODP)