[#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 like 'Showcase'.

I think just limiting it by name to 'Apps' will add to confusion if people add other coverage, or even half-complete hackathon entries. Unless we make the extension showcase, but give people the config option of how it should be on their site; or create the namespace data.example.org\{apps,ideas,coverage,showcase} etc.?