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
log the notebook server directory

suggestion provided by @Carreau
make SIGUSR1 and SIGINFO trigger printing of info

Directory and url/port information for the notebook server is now
printed when it receives one of these signals. SIGINFO is available only
on BSD-based systems, and is triggered via Ctrl-T.
print info string on interrupt, log it on startup
Merge pull request #3033 from ivanov/print-server-info

on resume, print server info again