Generate sitemaps.xml and provide a mechanism to re-hash the sitemap

[bmcminn]

Need a method to generate the sitemap.xml file.

• XML Sitemap Guidelines - http://www.sitemaps.org/protocol.html
• Need a way to specify priority - global config.php setting per content type (page, article, etc)
• Consider using underscore.php - http://brianhaveri.github.io/Underscore.php/