Store spatial label on the Swedish profile

Added example and tests
Don't display harvest sources in home page widgets
Add test for _object function
[#1] Add plugin interface for handling downloads

A new `IRDFDCATHarvester` interface offers two methods:
`before_download` and `after_download`, which get passed the url and the
file contents respectively. Returning None on either will stop the
gather stage. Useful eg for validations.
Preliminary integration with the RDF validation service