Support automatic cookie removal on logout

Support of cookie destroying on logout by adding "remove_cookies: { 'cookieName1', 'cookieName2':  { 'path': '/', 'domain': null } }" in your configuration e.g.

```
$app['security.firewalls'] = array(
[...]
		'logout' => array(
			'logout_path' => '/user/logout',
			'remove_cookies' => array('cookieName1', 'cookieName2' => array('path' => '/', 'domain' => null))
		),
[...]
);
```

Based on discussion https://groups.google.com/d/topic/silex-php/828UcSi7bks/discussion