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 (see also #178 and particularly the last comment, things changed since the description was done)
* [x] #178 for default behaviour
this is not about registered subscribers, but about subscibers the constructor should autoregister.
are we ok with the name? or does somebody have a better name? @lolautruche wants a simple name (and i agree) because people can overwrite this method to add their own subscribers.