Watches a specified iCal-format calendar for events. When the time
for an event comes, sends the name of the event to a servant that
supports the "commandable" role
(http://belphanior.net/roles/commandable/v1).

For example, say you have an iCal at www.foo.com/calendar that has an entry
named "wake me up" at 10AM today. If you have a servant at 127.0.0.1:3000
that supports "commandable" and understands the "wake me up" command,
you could run the calendar_watcher_servant with a servant_config file
that looks as follows:
{
  "ip":"127.0.0.1",
  "port":"4000",
  "calendar url":"http://www.foo.com/calendar"
  "servant url":"http://127.0.0.1:3000"
  "update seconds":"60"
}

With this configuration, the calendar watcher servant will check the
calendar every 60 seconds for new events. When it checks after 10AM,
it will discover the "wake me up" event and send it to the servant at
127.0.0.1:3000.