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/folder/IMAP.py

@@ -1,5 +1,5 @@
 # IMAP folder support
-# Copyright (C) 2002-2012 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
@@ -20,14 +20,13 @@ import binascii
 import re
 import os
 import time
+import six
 from sys import exc_info
 
 from .Base import BaseFolder
 from offlineimap import imaputil, imaplibutil, emailutil, OfflineImapError
 from offlineimap import globals
-from offlineimap.imaplib2 import MonthNames
-
-import six
+from offlineimap.virtual_imaplib2 import MonthNames
 
 
 # Globals