New in 1.3:
* Don't remove empty mailboxes.

New in 1.2:
* Check that an output box is actually specified
* Fix --read-only typo in pod docs, thanks Neil McGovern

New in 1.1:
* Workaround a bug in Mail::Box that was preventing maildirs 
  from working.
* Default to the current time if a timestamp is not present.
* Accept multiple mailboxes on the command line, not just one.
* Close the source mailbox now.
* Modified patch from Cyprien <cyprien@cypou.net> to preserve 
  timestamps on the input mailbox. 
