Use 'reference' value when creating an IMAP directory

A repositories 'reference value is always prefixed to the full folder
path, so we should do so when creating a new one. The code had existed
but was commented out since 2003, I guess the "reference" option is not
too often used.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/IMAP.py

@@ -1,6 +1,5 @@
 # IMAP folder support
-# Copyright (C) 2002-2007 John Goerzen
-# <jgoerzen@complete.org>
+# Copyright (C) 2002-2011 John Goerzen & contributors
 #
 #    This program is free software; you can redistribute it and/or modify
 #    it under the terms of the GNU General Public License as published by