[#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).
I really think this is best placed as separate extension to start with, specially during early development, as it allows much faster iterations and does not tie it to core releases. As a general rule I think we should keep core as lean as possible, with really good test integration with extensions (like @seanh suggestion of running extensions tests as part of the core suite). But this is a separate discussion.