Add hash invalidation on logout

Add a LogoutHandler to use with Symfony security component to ban hash request when a user logout

see #68
in a general case, it could. But not for the SecurityBundle. It would even hurt: given that the order of firewall is important and adding an extra firewall in the wrong position could create security issues in your app, the bundle requires all firewalls to be defined in the same config files (next config files can overwrite the config of known firewalls, but they cannot add new ones). If you prepend a config, you will need to define all firewalls expected by the user in such case, which gets insane.