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).
> It isn't immediately clear that 'Related Items' refer to examples of data reuse.

Is a newspaper article quoting the data a 'reuse'? Surely that's just a 'report'. Is a citation in a paper a 'reuse'? Probably not. Do we want to include these in our Showcase? Probably. I still think that 'reuse' is jargon that few will 'get'. Maybe 'related' is better...