saml2: Add a class for SAML2 constants.

This patch adds the SimpleSAML_XML_SAML2 class, which currently contain
SAML 2 status codes constants. New SAML2 constants should be added to this
class.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1569 44740490-163a-0410-bde0-09ae8108e29a