Pure Scala msgpack #26

I'm now prototyping pure-scala implementation of msgpack for supporting Scala, Scala.js, and Scala.native:
wvlet/airframe#127

Basic ideas for the implementation is as follows:

• Having SPI for providing interfaces for implementing MessagePack based codec (unpackXXX, packXXX). This will be useful if you need to implement MessagePack based object serizliation/deserialization code.
• Remove performance specific optimization in msgpack-java (e.g., MessageBuffer)
• Use simpler Value classes (e.g., Integer, Float, String, Array, Map, etc.) suited to Scala case classes. msgpack-java needed to use complex interface hierarchies to support immutable classes. I want to remove this complexity.