introduce a virtual imaplib2

The virtual imaplib2 first try to import imaplib2 when provided by the system.
If not provided or if the version is not supported, fallback on the bundled
imaplib2 version.

Distribution maintainers can now easily remove the bundled imaplib2 version if
they want to get it packaged outside of offlineimap.

We still want to provide imaplib2 by default because:
- this library is neither in Python core nor packaged by a lot of distributions;
- users expect to be able to run offlineimap by just downloading the tarball or
  after a git clone.

In order to avoid unexpected (too old) versions of imaplib2, we restrict the
supported versions of this librabry.

Reviewed-by: Łukasz Żarnowiecki <dolohow@outlook.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaplibutil.py

@@ -1,5 +1,5 @@
 # imaplib utilities
-# Copyright (C) 2002-2015 John Goerzen & contributors
+# Copyright (C) 2002-2016 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
 #    the Free Software Foundation; either version 2 of the License, or
@@ -23,12 +23,12 @@ import threading
 from hashlib import sha1
 import socket
 import errno
+import zlib
+import six
 
 from offlineimap.ui import getglobalui
 from offlineimap import OfflineImapError
-from offlineimap.imaplib2 import IMAP4, IMAP4_SSL, zlib, InternalDate, Mon2num
-
-import six
+from offlineimap.virtual_imaplib2 import IMAP4, IMAP4_SSL, InternalDate, Mon2num
 
 
 class UsefulIMAPMixIn(object):