get-my-ip.py: Add a script for getting your public IP address

This will help students connect to each other on the LAN during class.
Between this script and [git daemon][1], it should be fairly easy for
students to share their work via Git during a boot camp, assuming that
the LAN is friendly enough (e.g. no filtering 9418).

If you know that everyone will be using Linux, you can probably skip
this script and just have students use ifconfig and/or iproute2
instead.  I'm not sure what the recommended method is for other OSes.

[1]: http://www.kernel.org/pub/software/scm/git/docs/git-daemon.html