Respect XML namespaces in DTD tag name construction.

DTDs do not understand XML namespaces and requires the full qualified
name be used if the tag is defined with a namespace prefix. Construct
the qualified name if a namespace is attached to a DOM node.
Restore xml_security.rb.

libxmlsec1 is strict with trust chain verification. Until it's possible
to easily trust a cert, this causes a problem for integration tests with
service providers where the CA is not easily accessible (eg if the SP CA
is firewalled off from the IdP server). Reverting to xml_security.rb
allows us to ignore certificate chain verification.
Use xml_security to verify signatures. Add logging.

  * Add logging into the gem so we can inspect what's happening.
  * Use xml_security instead of libxmlsec1 to do verification
    so we can be looser with certificate chain requirements.
  * Fix namespace bug in xml_security.rb that causes lookup failure of
    the signature node.
Merge pull request #6 from omb-awong/back_to_xmlsecurity

Make signature verification more loose with trust chain verification.