AlarmPoller: basics for notifying only when things change.

Prepare to log when the alarm state changes. Also log whenever the alarm
goes off, but only once (so don't *keep* logging as long as it's in
triggered).
Basic logging support

Use the python built-in logging system.

Stil todo:
 - Configurable log level (command line option should suffice)
 - Extend format to include timestamps
 - Log to syslog?
Handle connection loss

If we lose the connection, log it and try to reconnect.  Ensure that we
sleep between connection attempts so we never end up hammering the PLC
with connection attempts.