saml2: Add a class for SAML2 constants.
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.

sgomez/saml2,sitya/saml2,ArtemTeleshev/php-saml2,madmatt/saml2,newmen/saml2,cb8/saml2,simplesamlphp/saml2