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
Not sure I'm a fan of all the punctuation in L16. What about...

```ruby
self.content = "#{content} #{xml_keyword}".strip
```