[#64] Showcase specification

Please use this Pull Request for discussion related to the new CKAN 'Showcase' feature (aka, Related Items). **DO NOT MERGE** until the spec has settled (likely when the feature is released).
Sure, "Related Items" hasn't caught on. "Showcase" seems ok to me, but it is a little bit abstract. You can't ignore that "Apps" is by far the most popular word people have chosen. It might not cover well the edge cases of newspaper articles and blob posts, but actually no-one seems to be cataloging these anyway. So I think it needs a damned good reason to not call it "Apps".