Provide public API functions

Currently, other modules relying on this one require access to the global variables provided by the library if they want to extend SAML functionality. It would be much cleaner if this Drupal module provided wrapper functions for these so that callers wouldn't have to access the global variables directly.

For now, two valuable functions would be:

• simplesamlphp_auth_is_authenticated()
• simplesamlphp_auth_get_attributes()