Adds logout & logout.post events

Moves the actual logout actions to the User service to be able to trigger pre & post logout events.

Usage:

```
// ABOUT TO BE LOGGED OUT
$sharedEventManager->attach(
            'ZfcUser\Service\User',
            'logout',
            function (\Zend\EventManager\Event $e) {
                $user = $e->getParam('user');
            }
        );
```

```
// HAS ALREADY BEEN LOGGED OUT
$sharedEventManager->attach(
            'ZfcUser\Service\User',
            'logout.post',
            function (\Zend\EventManager\Event $e) {
                $user = $e->getParam('user');
            }
        );
```