Messagepack support.

You will need this Messagepack library: https://github.com/msgpack/msgpack-javascript

Usage:
```
<script src="transit-0.8.714-min.js"></script>
<script src="msgpack.js"></script>
<script src="transit_with_msgpack.js"></script>
<script>
    var reader = transit.reader("msgpack"),
        writer = transit.writer("msgpack");
    reader.read(writer.write(2));
</script>
```
In response to: https://github.com/cognitect/transit-js/issues/6