Fix 590

This branch (which I stupidly made out of auto-tuning, ignore that stuff ;-) should make URLs in feeds full, absolute URLs, as required by the RSS spec.

While most RSS readers are smart enough to figure out this when using the feed from the site's domain, it all breaks when using feedburner.

This may have a bad side effect in that a wrong configuration or a bug in the handling of SITE_URL and BASE_URL will break things.