Currently, PySpark bundles all of its library dependencies (only Py4J for now) and users don't have to install any extra packages.  Would `msgpack`'s license permit us to bundle a .zip or .egg package in the PySpark lib/ directory?  Does it use C extensions that would preclude this?

If we don't bundle msgpack, then I think we could wrap this import in a try-catch and log a warning if msgpack isn't installed.  We do something similar for `numpy` in the sampling methods, if I recall.