Futures on responses

We've done a bit of musing on how we would use [`concurrent.futures`](https://docs.python.org/3/library/concurrent.futures.html) within a Python 3 version of multyvac/cloud.

This would mean that our underlying network client would need to use either:
- [Requests futures](https://pypi.python.org/pypi/requests-futures)
- [Tornado's Async HTTP Client](http://tornado.readthedocs.org/en/latest/httpclient.html)