Adding support for LogoutRequest to response.rb

  * Support sso_url in metadata.
  * Add building for LogoutRequest.
  * Force all acs_urls and sso_urls to come from metadata. Avoids a
    MiTMable configuration.
  * Renames saml_acs_url to saml_response_url where it's being used in
    a request-type agnostic manner.
  * Force signature validation for LogoutRequests. Unlike AuthnRequests,
    forging one can use ambient credentials to DoS the user's session.
  * Hack up the specs to override the fingerprinting/certs used for
    golden files. Not ideal but good enough for now.
  * Refactor all the signing logic into one place.

Moving forward, xmlsecurity should just be replaced with the
nokogiri-xmlsec-me-harder.
Start implementing LogoutResponse generation.

  * Create a basic LogoutResponse builder
  * Expose it in the Contoller mixin.
  * Begin writing a spec for it.