Updated RSS feed

Apologies to those people seeing a string of old posts from me
in their RSS readers. I’ve updated my RSS generating code to
(hopefully) correctly handle ETag and Last-Modified headers and to
return a 304 if no changes have been made. I’ve also taken the
opportunity to change my permalinks so that they point to one page
per post. All old permalinks should still be supported. This will
allow me to add-in per post tracking, etc. If anyone spots a
problem, please e-mail me and let me know. Thanks.