First pass at basic /registry/<package>.rss route.

Solution for #121 

This is a first pass, mostly to try playing with newww. It's a feature I think would be really useful. Not sure long term support or issues with it, but here's a first blush.

The registry facet gets a new route, `/registry/{package}.rss`, that returns an rss xml with reach release as an `<item>`. Long term, I'd like to see options for "deep" notifications, generating items for each dependency update as well. This way, you could subscribe to the "master" rss feed for your package, and know when your deps are updated to run tests or re-release or whatever.