Removed unused deserialize_state_is_ready()
Removed unused serializer_yajl_state_free()
Integrate supertype of deserializer_state

This patch integrates the supertype of the `struct deserializer_state`
into the `struct deserializer_state` itself.
Integrate deserializer_state.{c,h} into other source files

(into deserializer_callbacks.{c,h} and deserializer.{c,h})
Integrate string_jump_state into deserializer callback code
Merge pull request #412 from matthiasbeyer/refactor-json_deserializer

Refactor JSON deserializer