Add handler for getting external IP address
Add dnsjava as external library

Will be used to make DNS queries, which in turn are used to get the
client's external IP address.
Retrieve external IP address for client

I dunno...might be useful. Kind of fun to learn how to make DNS queries
in Java, anyway. This code is the equivalent of doing

    dig myip.opendns.com @resolver1.opendns.com

which is a quick and easy way to get one's external IP address.