Add Rostock harvester for original portal field

For a better dataset attribution I have created another harvester: a
separate Rostock harvester which fills the metadata original portal
field. This way it is more easier to map datasets to their original
source in case of manual updates.

This fixes fraunhoferfokus/GovData#51.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>