readPayloadAsReference

Implemented #105 (returning a reference to the message buffer). Currently it simply returns a slice of MessageBuffer to the user. It is orders of magnitude faster than `readPayload(byte[])` since it can avoid the cost of buffer allocation and memory copy: 

![1__leo_weaver_local___work_git_msgpack-java__zsh_](https://cloud.githubusercontent.com/assets/57538/3226967/72d315cc-f075-11e3-8ab9-f0f3edb0a7a6.png)

Future work:
- To mange the total amount of allocated memory, MessageUnpacker should have a buffer allocator.