Feature/support metadata extension mdui

Add models to handle SAML MDUI specification per http://docs.oasis-open.org/security/saml/Post2.0/sstc-saml-metadata-ui/v1.0/sstc-saml-metadata-ui-v1.0.pdf
![](http://silencenogood.net/wordpress/wp-content/uploads/2012/07/disco-stu.gif)