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).
I agree with the stages here, I'm happy with the "scope" that they lay out for this first pass at implementing this extension.

Just want to comment that stage 3 could be either a standalone site (which is my strong preference) or it could remain as a within-a-single-CKAN-site feature, but the point is that it has all the "community moderation" features that would be necessary to allow your community of users to post showcase items and have them show on the site immediately without having to be moderated first, while also having some community-based mechanisms to keep spam down, let dataset owners control what appears on their own dataset pages, etc.

In stage 2 we avoid this complicated and difficult problem by having any submissions from users go into a moderation queue (of some sort) where they have to be approved by a trusted moderator user before they are published anywhere on the site.

Stage 3 is quite complicated imo and definitely out of scope for this first implementation, we are definitely aiming for stage 1 first and then *maybe* a version of stage 2, for now.