unicode is not a valid exception class

The error was: exceptions must be old-style classes or derived from
BaseException, not unicode
Fix AuthnContext retreival.

AuthnContext is not a SAML2 attribute, but is instead
available in the gathered session_info.
Request required AuthnContext from IdP.

This change depends on a feature in a yet unreleased version of pysaml2.
Rename functions and add documentation.