Add support for jsonapi.org format.

Hello,
We're using this gem within the standards written on jsonapi.org. I've added support for this. Just add in the class declaration:

```ruby
class User
  include_root_json true, format: :jsonapi
  parse_root_json true, format: :jsonapi
end
```

Could you take a fast look and tell me if something should be changed? It works, but I don't feel the code very clear.