Plugin ported to VRaptor 4

As VRaptor 4 already uses Gson as the Json serialization engine, I just kept the @Json annotation and the interceptor to give the developer a 'shortcut' to trigger the serialization.

So, instead of writing something like:
```java
public void list(){
    result.use(Results.json()).withoutRoot().from(dao.findAll()).recursive().serialize();
}
```
You just annotate the method with @Json and let it return your Java objects, like:
```java
@Json
public List<Person> list(){
	return dao.findAll();
}
```