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 agree about 'reuse'. I don't like it, but _for now_ it's useful so we're all talking about the same thing. I still have reservations about how specific 'App' is.