Install a `pyzmq` package compiled against `libzmq3`. Fixes #57.

Although Debian experimental has `libzmq3`, it does not have `pyzmq` compiled against it. We install `pyzmq` from `pip` having `libzmq3` already installed.
Install pyzmq from pip after salt being installed. Override current pyzmq.
No user input using apt.
Warn Debian users about installing PyZMQ from PyPi.
Only add Debian experimental repository if not yet added.
Because of PyZMQ need to be compiled against ZMQ3, add the pip allowed checks.