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.