Msgpack object stream

Added msgpack object stream decoding. This allows RPC calls which return a body to be handled properly. In this change, I switched from `umsgpack` to [msgpack-python](https://github.com/msgpack/msgpack-python) since getting object streams working under it seemed pretty easy.

Added a new test and updated the existing ones, since the return format of `serfclient.SerfConnection.call()` changes to `Tuple(header, body)`

One thing to note here is that I tested this under two different msgpack-python versions: the first being the Fedora package version from their base YUM repos, which was 1.x. This didn't work, but installing 0.4.1 from `pip` works great. I updated this in `setup.py`.