[#19] Move existing implementation from ckanext-sweden

This includes:

* A parser class that can be used to parse RDF documents into CKAN
  dataset dicts (includes a CLI)
* A base profile which supports all the DCAT fields
* A harvester that integrates the RDF parser to import remote RDF
  metadata into CKAN
[#19] Refactor harvesters code

To accommodate the new preferred RDF based harvester, separate the
depecrated XML one dependencies and make the code generally easier to
follow.

Also renamed the RDF harvester to `DCATRDFHarvester` to follow the
existing pattern.
[#19] Migrate and set up test suite and examples
[#19] dcat:landingPage, not landingUrl
[#19] Fix parsing of DCT.format on distributions

DCT.format refers to the `format` function. Changed to DCT['format'].
Tests.
[#19] Add JSON-LD example and rdflib-jsonld requirement

Example is based on @philipashlock's one in
project-open-data/project-open-data.github.io#309
[#19] Update documentation

Docs about DCAT to CKAN mapping, RDF Parser, profiles and harvester.

Moved to markdown to take advantage of the nice GitHub rendering
features.
Merge branch '19-rdf-parsers'