Showcase specification

This Pull Request refers to issue #64.

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 was trying to see what Mozilla WebMaker calls things, because it lets you make a web page, a video, an app, etc. For the most part they seem to studiously avoid using any one noun to refer to all of them, but occasionally they call them "makes". And is a verb it's "making" obviously and the people are "makers", and there are also sometimes phrases like "make things|something for the web" ("make something with open data").