Supervisor

This is the first step towards agent auto-updates. The supervisor
regularly checks if the agent is still running and, if not, restarts it.

If it finds an update in an update drop directory specified on the
command line, it will apply it before starting the agent.

Currently, applying an update just means unpacking a zip file to a
specified location.
TODO for later: use config for default value

For now, change it to `join(expanduser("~"), ".pyfarm", "agent_updates")`