custom AuthComponent class support added

[spandansingh]

With this the user can override Configure::Opauth['AuthComponent'] for their own custom AuthComponent Class.

eg :- If user want to make his own AuthComponent class like

class MyAuthComponent extends AuthComponent{

}

then in his bootstrap file he can write CakePlugin::load('Opauth', array('routes' => true, 'bootstrap' => true)); Configure::write('Opauth.AuthComponent','MyAuth') ; and OpauthController will be allowed by MyAuth