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 think 'reuse' as a noun is pretty clumsy and is jargon - who talks about it apart from open data insiders? I was keen on other words until I saw your pretty convincing data about everyone using the word 'App'.