[#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).
We normally make features a core extension if we anticipate being used by a large chunk of sites. I think that fits the criteria. I know @rossjones is keen for it not to be core because it means it ties it to those releases, but I think that being an extension it is no trouble to copy it onto one's own CKAN fork in that case, and CKAN gains a lot from this feature being in the default configuration. The long tail of smaller sites will more likely benefit from this feature.