[Review] Request from 'digitaltom' @ 'digitaltom/ruby-saml/review_130904_allow_for_settings_authn_context_and_settings_authn_context_decl_ref'

This adds support for a AuthnContextDeclRef element inside RequestedAuthnContext. 
It looks like this: 

```xml
<samlp:RequestedAuthnContext>
  <saml:AuthnContextDeclRef>suse/name/password/uri</saml:AuthnContextDeclRef>
</samlp:RequestedAuthnContext>
```

We need that for choosing a login page on our saml provider on sp initiated requests. 
Instead of 'suse' use 'http://example.com/class_ref'
(review the test)