Start to write link checker code

For the upcoming link checker I have added files for the unit tests and
the link checker class.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Write methods for URL validation and dataset check

Just added more tests to cover the functionality, especially special
cases like follow location.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Revert configured path for the logging

Accidentally, I changed the path for the logging file. Hence, this
commit.

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