[WIP] Add JSON deserializer yajl callbacks

This PR adds the JSON deserializer yajl callbacks with implementation.

Please review and tell me what is missing and what can be
optimized here.
No I seriously have to scold you.
I just had a look at the `yajl` headers while performing updates and it __explicitely__ states that, if the number callback is set, _neither the integer nor the double callback_ is called!
Read the documentation more carefully next time!