Write session data to a log

[ecridge]

Make IceTimer output ice-time, total time and total number of teams to a log after each session.

The easiest way to implement this will probably be to write the log when the program is terminated, as this will be immune to restarted sessions (e.g. due to a change in the number of teams) and should work regardless of how the program is exited.

It will probably be useful to group terms into different log files so the writer should check for large gaps between sessions.