[#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).
I don't get "Show & Tell". And it's not naturally a subset of a "Showcase". I think there is a reasonable convention about talking of "featured content", so how about "App" and "Featured App"?