[#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