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).
Again, I think we can leave this to individual sites to override templates to choose what gets displayed. Or it could be built as a separate plugin within the same extension that adds the features you discuss