In order to unpack streaming data, it is necessary to receive and store some data. 
For that purpose, it is necessary to hold unpacker as an instance variable. 
But instance variable can not be define in category.
```
@implementation MessagePackParser (Streaming) {
    msgpack_unpacker unpacker; /// complie error.
}
```

I think there is two options.
1) define new class and my change move. (ex: MessagePackUnpacker.*)
2) define on MessagePackParser (my original idea)

Streaming deserialization function is standard function of messagepack.
Therefore, I think that it is very natural. 

Do you have any idea ?