Added site map generation support for plugins via events (hooks).
Added Google\SiteMap class which offers basic functionality to build site maps.
Added Profis\CMS\SiteMap class which extends Google\SiteMap class and offers functionality to add pages to the site map.
Modified api 'sitemap' request to use Profis\CMS\SiteMap class.
Added 'nositemap' field to pages table in database schema.