Add few comments

Just added a few comments to the harvesters.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Add file handler to logger

Error messages should be logged so problems with the remote data can be
evaluated afterwards. Hence, I have added a file handler to the logger
which logs the outputs of the GovData.de harvesters to a file.

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