Move Authorization and Authentication plugin structures into core, instead of as plugins.

This move makes sense as you can addPlugin('Authentication') for example - these are abstract classes designed to be implemented, not used directly.

plugins/ReverseUsernameAuthentication/ReverseUsernameAuthenticationPlugin.php

@@ -31,8 +31,6 @@ if (!defined('STATUSNET') && !defined('LACONICA')) {
     exit(1);
 }
 
-require_once INSTALLDIR.'/plugins/Authentication/AuthenticationPlugin.php';
-
 class ReverseUsernameAuthenticationPlugin extends AuthenticationPlugin
 {
     //---interface implementation---//