Add support for alternative message date synchronisation

Global or per-repository option utime_from_message tells OfflineIMAP
to set file modification time of messages pushed from one repository
to another basing on the message's "Date" header.

This is useful if you are doing some processing/finding on your
Maildir (for example, finding messages older than 3 months),
without parsing each file/message content.

From: Cyril RUSSO <boite.pour.spam@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/IMAP.py

@@ -15,14 +15,13 @@
 #    along with this program; if not, write to the Free Software
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
-import email
 import random
 import binascii
 import re
 import time
 from sys import exc_info
 from .Base import BaseFolder
-from offlineimap import imaputil, imaplibutil, OfflineImapError
+from offlineimap import imaputil, imaplibutil, emailutil, OfflineImapError
 from offlineimap import globals
 from offlineimap.imaplib2 import MonthNames
 
@@ -431,21 +430,12 @@ class IMAPFolder(BaseFolder):
         :returns: string in the form of "DD-Mmm-YYYY HH:MM:SS +HHMM"
                   (including double quotes) or `None` in case of failure
                   (which is fine as value for append)."""
+
         if rtime is None:
-            message = email.message_from_string(content)
-            dateheader = message.get('Date')
-            # parsedate_tz returns a 10-tuple that can be passed to mktime_tz;
-            # Will be None if missing or not in a valid format.  Note that
-            # indexes 6, 7, and 8 of the result tuple are not usable.
-            datetuple = email.utils.parsedate_tz(dateheader)
-            if datetuple is None:
-                #could not determine the date, use the local time.
+            rtime = emailutil.get_message_date(content)
+            if rtime == None:
                 return None
-            #make it a real struct_time, so we have named attributes
-            datetuple = time.localtime(email.utils.mktime_tz(datetuple))
-        else:
-            #rtime is set, use that instead
-            datetuple = time.localtime(rtime)
+        datetuple = time.localtime(rtime)
 
         try:
             # Check for invalid dates