Freeze versions for the PIP requirement file

It seems to be the better practice to set defined version numbers for
the PIP dependencies.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Encapsulate remote iteration of datasets into util

The iteration of remote content has now been encapsulated into its own
utility method. Using the generator interface the datasets can now be
easily iterated.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Add link checker to the commands

Just added the link checker to the commands as well.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Write tests and add functionality for redis usage

The link checker results will be stored in Redis, which is a simple
key-value store. For this I have added the dependency and started to
write tests.

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