[#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 want to focus too much on the naming discussion. One think is clear from @brew's great report is that people call it all kinds of things, so perhaps I will go with a generic name (my vote goes to "uses" or "reuses", with some of them being "featured"), and then make it as easy as possible for users to change that to whatever they like.

Also another thing to consider is that "Show & Tell" is a well established concept in English but that might be hard to translate to other languages.