Add normalize extras function

In order to deal with the extras in a generic fashion I have added a
normalize extras function. Its specification is encoded in the unit
test I have added.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Add normalize action dataset function

Obviously, the Model API /rest is too slow for remote validation. Hence,
I will use the Action API which then again needs normalization in order
to be validated against the JSON Schema. I have added this function and
the appropriate test cases. In addition, I had to extend the normalize
extras function so it normalizes string encoded strings, as well.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Start to implement validator command

I have implemented the first version of the validator command. The
validation works for remote CKAN instances, uses the Action API and
prints the broken rules.

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