Adding a @Json annotation

Hello,

I suggest adding a `@Json` annotation for controllers in order to render the JSON-encoded data to users, which would be used like this:

```php
<?php
    /** 
     * @Route("/test")
     * @Json()
     */
    public function testAction()
    {   
        return array('something' => 42);
    }   
```

And rendered as follow with the good headers and kernel charset :

```
{"something":42}
```