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/repository/Gmail.py

@@ -36,6 +36,13 @@ class GmailRepository(IMAPRepository):
                                 'ssl', 'yes')
         IMAPRepository.__init__(self, reposname, account)
 
+        if self.account.getconfboolean('synclabels', 0) and \
+              self.account.getconf('status_backend', 'plain') != 'sqlite':
+            raise OfflineImapError("The Gmail repository needs the sqlite backend to sync labels.\n"
+                                   "To enable it add 'status_backend = sqlite' in the account section",
+                                   OfflineImapError.ERROR.REPO)
+
+
     def gethost(self):
         """Return the server name to connect to.
 
@@ -71,4 +78,3 @@ class GmailRepository(IMAPRepository):
     def getspamfolder(self):
         #: Gmail also deletes messages upon EXPUNGE in the Spam folder
         return  self.getconf('spamfolder','[Gmail]/Spam')
-