Adding SAML based Authentication

Depends on https://github.com/SURFnet/Stepup-Middleware-clientbundle/pull/7
Depends on https://github.com/SURFnet/Stepup-Gateway/pull/4
Depends on https://github.com/SURFnet/Stepup-saml-bundle/pull/4
Depends on https://github.com/SURFnet/Stepup-Middleware/pull/18
Did you skip `@var`s because of the PHPMD bug?