Put the dataset url into HarvestObject.guid

There's no other place to put it. We can't use harvest_object.source.url,
because that's the Harvest source URL (which is unique for each harvest). We
can't change it for each related package, unfortunately.
Use the URL's sha1 hash as ID, so we limit its length
Set dataset's groups to be the ones passed in config, if existent

Now, you could add in the configuration part of the harvest source:

  { "groups": ["brasil", "paraiba"] }

And every dataset loaded by the harvester will be added to both groups.

Also, I added an extra source_url field to the datasets, pointing to the Iota
URL that it came from.