Introduce supervisor daemon for pyfarm-agent

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.
Fix copy-paste error
Change default updates drop dir
Drop unneeded log statement
Make main loop interval configurable
Fix some log strings
Wrap actually updating the agent in a try-block
Forgot needed import
Merge pull request #74 from guidow/supervisor

Initial Supervisor