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

Please review the following changes:
  * b5c0b06 allow for settings.authn_context and settings.authn_context_decl_ref
  * a968531 add :authn_context_decl_ref settings option