support use_cloudpickle in addition to use_dill

In the same manner as `use_dill`, this adds support for PiCloud's `cloudpickle` from their [`cloud`](https://pypi.python.org/pypi/cloud) library. It's by far the most reliable and wtf-free way to do remote code execution in python that I've found. I'm using this hack in some of my own projects and I figured others may as well benefit.