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).
> Examples of data use may be submitted for inclusion by anyone who visits the site, and selected for inclusion by 
> site maintainers

This is very clear and I think a good aim / restriction for (at least) a first version of this extension.

Examples of data use -> Showcase items, though.