Should JsonObjectStream expose a reference to JsonBuffer?

To create an array of array the method JsonBuffer::createArray() is required: 

```
DynamicJsonBuffer buffer;
JsonArray& list = buffer.createArray();
JsonArray& item = buffer.createArray();
item.add(1);
list.add( item );
```

However, the class JsonObjectStream does not a reference to the underlying JsonBuffer. If this is desired, I will provide a patch.
