Changes in Files:
* Sitemap.php - controller for the sitemap, new
* Sitemap.ss - template for the sitemap, new
* main.php - added director rule
* SiteTree?.php - added 'priority' database field, cms code to override priority
TODO: ping google when the sitemap is updated,
namely when pages are added / removed, or when a priority is overridden
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@41689 467b73ca-7a2a-4603-9d3b-597d59a354a9