Rr/rss

**I've made some major changes since I created this pull-request. You'll have to do a diff against rr/rss, instead of relying on the pull-request diff**

* RSS feeds for content-items and topics. 
* There are some broken non-rss tests; these are in a separate pull-request.

* To download with wget:

        $ wget -nv localhost:3000/en/site/images/list.rss
        $ wget -nv localhost:3000/en/site/images/list.rss\?updated_since=2013-10-30T13:30:47+13:00