Fixes #10: Fixed sitemap "loc" element to have encoded entities.
Fixes #9:  Honor the language setting if other plugins set the language.
Fixes #12: sitemap.xml will now redirect to sitemap_index.xml.
Pull #8: Added filters 'wpseo_sitemap_exclude_post_type' and 'wpseo_sitemap_exclude_taxonomy'
to allow themes/plugins to exclude entries in the XML sitemap