Replace GovData.de source by static file dump

After GovData.de provides now a daily static dump I have adjusted the
source to use this dump instead. Unfortunately, it seems that zipruby
cannot open gunzip files and zlib cannot open zip files. Hence, I have
added code to distinguish the file extension in order being able to
decompress both file formats.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add minimalistic usage description to the README

Just added some simple usage notes to start the harvesting process and
listing the basic requirements needed to do so.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>