This feature filters users who may log in via CAS. This is useful when
both CAS and password authentication is enabled and there is a mismatch
between some GNU social account names and CAS user names. This prevents
CAS users from logging in as someone else on GNU social.
Fixes handle()-related strict warnings such as "Strict Standards:
Declaration of AdminPanelAction::handle() should be compatible with
Action::handle()"
Ref. #190
lib/plugin.php now has a parent onAutoload function that finds most common
files that are used in plugins (actions, dataobjects, forms, libs etc.) if
they are put in the standardised directories ('actions', 'classes', 'forms',
'lib' and perhaps some others in the future).
