[#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).
Naming has certainly been tricky. I agree there are problems with Show & Tell as a name for individual reuse examples. I was trying to encourage people to share their reuse examples. Then the best would be featured in the Showcase. I'm not sure this is very clear.