Pass AuthnContextClassRef into MELLON_ variables

[szabogyula]

It should be very useful to pass the AuthnContextClassRef value from saml assertion to any MELLON_ env variable.

[thijskh]

I'm guessing you already know that Mellon can already authorize logins based on the AuthnContextClassRef itself? I can understand that there are some use cases to pass it to the environment though. A pull request to this effect would certainly be welcome.