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
Bit of a hack using this form of string concatenation / automatic coercion, but this also doesn't do what you'd expect on first reading it. For random words `a b c d e f g h` the resulting string would be: `a b c d e fg+h` &mdash; missing a space.

What do you think about:

```ruby
extra_keyword = Faker::Lorem.words(2).join('+')
Faker::Lorem.words(6).push(extra_keyword).join(' ')
```