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).
Why would you pick a small sub-set of the apps to show in a site-wide showcase? Surely you want to list all the apps? Of course show some featured ones on the first page with some fancy graphics. But listing loads of them is a good thing for site owners. You show a nice big number of the count of apps, sending a clear message e.g. "we have 200+ apps that reuse our data".