Fix AL-level recognition.

Using the wrong reference to the accepted AL-levels resulted
in the IdP being unable to assert higher-than-default assurance
level.

Example log illustrating bug :

DEBUG Lowered authentication detected,
      'http://www.swamid.se/policy/assurance/al2' required
      ['eduid.se:level:2:200', 'eduid.se:level:3:300'],
      got 'eduid.se:level:2'
Support reverse-proxy configuration.

Add configuration parameter 'base_url' to support the
proxy being behind a reverse proxy (like Pound used for
HA and SSL/TLS termination), and not being able to figure
out the correct URL for use in redirects etc.