Rework example to summarise and extract all knowledge from NPO

[dbrnz]

Based on commits 6644f96, 0f5981f, and 2684b9c from napakalas:completeness-tracing:

1. extract_knowledge() should not be a KnowledgeStore method, as it is Npo specific.
2. Npo.extracted_knowledge() is really just for the example, so the code should be with the example and not in npo.py.
3. The example is best put into ./tools and modified to get sckan_version from the command line -- then it becomes a general purpose tool. Use argparse to provide CLI help.
4. As you will see, I've removed the connected_only parameters (and renamed the keys into knowledge), the idea being that the caller can filter out what they want to use.