Add SAML2_Response Processing

Add Response Processing Functionality to the SAML2 library, based on the [similar functionality from SimpleSAMLphp][1]

[1]: https://github.com/simplesamlphp/simplesamlphp/blob/3d735912342767d391297cc5e13272a76730aca0/modules/saml/lib/Message.php#L493