Added msgpack serialization support

I tried to resolve #48 by using [MsgPack 2.0 binary treatment of strings](https://github.com/msgpack/msgpack-python#note-for-msgpack-20-support).
However it seems that redis does not support those yet (sigh). I have opened https://github.com/antirez/lua-cmsgpack/issues/34 about it.
I believe this is still useful for those who want to reduce network IO since msgpack is smaller.
Once cmsgpack will support the binary data type we can flip the switch in a backwards compatible way.
All tests pass and documentation is added.