Make GmailFolder sync GMail labels

When synclabels config flag is set to "yes" for the GMail repo,
offlineimap fetches the message labels along with the messages, and
embeds them into the body under the header X-Keywords (or whatever
'labelsheader' was set to), as a comma separated list.

It also adds an extra pass to savemessageto, that performs label
synchronization on existing messages from GMail to local, the same way
it is done with flags.

We also introduce GmailMaildir repository that adds functionality to
change message labels.  It keeps track of messages modification time,
so one can quickly detect when the labels may have changed.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/IMAP.py

@@ -142,12 +142,15 @@ class IMAPFolder(BaseFolder):
 
     def _msgs_to_fetch(self, imapobj):
         """
-        Determines UIDS of messages to be fetched
+        Determines sequence numbers of messages to be fetched.
+
+        Message sequence numbers (MSNs) are more easily compacted
+        into ranges which makes transactions slightly faster.
 
         Arguments:
         - imapobj: instance of IMAPlib
 
-        Returns: UID ranges for messages or None if no messages
+        Returns: range(s) for messages or None if no messages
         are to be fetched.
 
         """
@@ -156,7 +159,7 @@ class IMAPFolder(BaseFolder):
             # Empty folder, no need to populate message list
             return None
 
-        # By default examine all UIDs in this folder
+        # By default examine all messages in this folder
         msgsToFetch = '1:*'
 
         maxage = self.config.getdefaultint("Account %s" % self.accountname,
@@ -194,7 +197,7 @@ class IMAPFolder(BaseFolder):
                     self.getrepository(), self, search_cond, res_type, res_data),
                     OfflineImapError.ERROR.FOLDER)
 
-            # Result UIDs are seperated by space, coalesce into ranges
+            # Resulting MSN are separated by space, coalesce into ranges
             msgsToFetch = imaputil.uid_sequence(res_data[0].split())
 
         return msgsToFetch