Creating WithoutRoot annotation to force JSON deserialization without root.

Related: #657.

As I said there, I think the best way is to create another annotation, because many user use something like `@Consumes("application/json")`, so if we add another attribute as @jayrmotta said, it doesn't work.

What do you think @lucascs @garcia-jj @Turini ?
@Turini if the merge is after 4.1.0.Final, wouldn't be better since 4.2(or the next version) ?