Moving the user context logic out of the kernel inheritance hierarchy.

fix #170

This adds an event handler to the cache kernel. The setup is:

```
$kernel = new HttpCache($kernel);
$kernel->addSubscriber(new UserContextSubscriber());
$kernel->addSubscriber(new PurgeSubscriber());
$kernel->registerSubscriber(new BanSubscriber());
```

**TODO**
* [ ] Adjust documentation
* [ ] #178 for default behaviour
indeed. i was looking at https://github.com/FriendsOfSymfony/FOSHttpCache/blob/master/src/CacheInvalidator.php but there it makes sense when you configure it as service and want to inject the symfony event dispatcher. will remove it here.