[#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 agree that 'App' is most associated with mobile applications, but I think there is also a reasonable association in people's minds with software generally, including web site services. Certainly on DGU people are have plenty of websites that people have added to our Apps.