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.
Remove xml_security.rb.

  * Use nokogiri-xmlsec-me-harder for certificate verification.
  * Replace original self-signed idp cert. The "X509v3 Authority Key
    Identifier" was causeing xmlsec1 to barf on "unable to get local
    issuer certificate"
  * Update golden files to use the new cert.
  * Clean up the fingerprint definitions and add some utility
    functions for signing/verifying xml documents
  * Fixup some badly written xpaths.
Make the valid_document function take a cert.

Accepting both certs and fingerprints removes a bunch of boilerplate.
Add xmlsec1 to travis and correct debugger gems.

This should fix Travis builds for ruby 1.9.3 and 2.x.
Replace named parameter with positional one.

Named parameters don't exist in ruby 1.9.3.
Merge pull request #3 from omb-awong/sso

Implement LogoutRequest parsing support for SingleSignOut.