Fixes handle()-related strict warnings such as "Strict Standards:
Declaration of AdminPanelAction::handle() should be compatible with
Action::handle()"
Ref. #190
Nickname verifications on registration and updates for profiles (not yet
groups) have been improved.
Minor bugs in RegisterAction were also fixed, where multiple forms would
be outputed because the function did not return after showForm(). This
will be solved more permanently with throwing exceptions in the future.
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).
