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.
This _still_ isn't resolved. You have also return if you are in either `STATE_INVALID` and in the default state, if `state->nboxbrackets == 0 && state->ncurvedbrackets == 0`!