Refactored sitemap generator to support RSS/Atom

* Refactored common logic into a Feed base class
* Added RSS & Atom support
* Added title / description extraction
* Added proper date handling
generate-feeds: added lxml bug note
devserver: simple local HTTP development server

This allows you to develop locally with non-minified stylesheets and scripts and
serve compressed resources in production without having to rewrite filenames or
use a templating system