Webdoc Refresh 2013

This is an overhaul of webdoc, including some major code refactoring, introduction of skins and plugins, and the addition of the new xamarin skin. This can be tested on the docs.xamarin.com server, located in /var/www/monodoc. Be sure to test all three skins (mono-external, xamarin-external, and xamarin-new). You can edit the skin in the plugins.def file.