the url to dictate arbitrary static method calls.
* Each xxx_rss helper has a single feed() call which takes an id as the argument
* xxx_rss::available_feedS() only returns feeds when they're applicable (ie
if you're viewing a tag, it won't show you an item feed).
* Feed urls are now in the module/feed_id form so that we can bind a
feed id to a given module
* Tightened up the Rss_Controller by using url::merge and some other tricks.
* Made the slideshow module express its own feed.
