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 ?
And the problem occurs on `GsonDeserialization`, so XML isn't affected, since XML use XStream.