on resume, print server info again

This commit makes it possible to differentiate between many different
long-running notebook servers, where the original ip address and port
information printed at the beginning has scrolled out of the screen.

We save the server location string that gets printed on startup, and
re-print it when the user attempts to interrupt the server with Ctrl-C

Thanks to @minrk for discussion on how this should work.

Also added a docstring to the start() method