Add Symfony2 stopwatch support

This PR fixes #40 
Actually I think that this is weird too because you simply assign the `Stopwatch` instance again that you just retrieved from the `StopwatchSubscriber`.

What do you think about using a placeholder for the `$stopwatch` variable? It simply doesn't really matter where that instance is coming from:

```php
// a Stopwatch instance
$stopwatch = ...;
$stopwatchSubscriber->setStopwatch($stopwatch);
```

By the way, is it really useful to provider a setter method for the stopwatch at all? I cannot think of a use case where you wanna do that. Just as a side note.