Add timeout.py: a script that alerts if a command doesn't finish in time.

Summary:
We can use this to replace the ad-hoc emailing we're doing in cron
jobs now when commands time out.  Instead, we can use alertlib to
email, send to hipchat, etc.  It will also make it easier to alert
when stuff times out inside shell scripts.

Test Plan:
Manually ran
   ./timeout.py --logs 2 grep asdf
and after 2 seconds got a prompt back, and did
   tail /var/log/syslog
and saw
   Apr 28 17:37:01 meta timeout.py: TIMEOUT running grep

Reviewers: chris

Subscribers: ben

Differential Revision: http://phabricator.khanacademy.org/D8594