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
Simplification, I can't see why it wouldn't as all LocalizedURI really adds is a lang attribute which seems to get redefined in logo anyway. Maybe they didn't want X > Y > Z starting from anyURI.