Include portalocker in imaplibutil.py

This patch include the usage of portalocker for Windows environments,
because fcntl is not available.

This patch is related to issue #37, issue 1.

offlineimap/imaplibutil.py

@@ -15,7 +15,6 @@
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 import datetime
 import os
-import fcntl
 import time
 import subprocess
 import threading
@@ -29,6 +28,14 @@ from offlineimap import OfflineImapError
 from offlineimap.ui import getglobalui
 from imaplib2 import IMAP4, IMAP4_SSL, InternalDate
 
+try:
+    import portalocker
+except:
+    try:
+        import fcntl
+    except:
+        pass  # Ok if this fails, we can do without.
+
 
 class UsefulIMAPMixIn:
     def __getselectedfolder(self):