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
The issue is that the kernel is not a builder object.

It is readable as long as you only call this method on it, but it becomes messy when you start chaining something else after the last ``addSubscriber`` call.
See http://ocramius.github.io/blog/fluent-interfaces-are-evil/ for a more complete explanation