add templateZipURI method to cache

This is sort of an implementation detail but it's useful to put the
logic for munging the tutorial on typesafe.com so all the HTML
presentation is on typesafe.com. So we want to get the raw zip.
We don't want typesafe.com to directly use a template cache.
Merge pull request #22 from typesafehub/wip/havocp-zip-uri

add templateZipURI method to cache