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/__init__.py

@@ -23,6 +23,7 @@ except ImportError: #python2
 from offlineimap.repository.IMAP import IMAPRepository, MappedIMAPRepository
 from offlineimap.repository.Gmail import GmailRepository
 from offlineimap.repository.Maildir import MaildirRepository
+from offlineimap.repository.GmailMaildir import GmailMaildirRepository
 from offlineimap.repository.LocalStatus import LocalStatusRepository
 from offlineimap.error import OfflineImapError
 
@@ -46,7 +47,8 @@ class Repository(object):
         elif reqtype == 'local':
             name = account.getconf('localrepository')
             typemap = {'IMAP': MappedIMAPRepository,
-                       'Maildir': MaildirRepository}
+                       'Maildir': MaildirRepository,
+                       'GmailMaildir': GmailMaildirRepository}
 
         elif reqtype == 'status':
             # create and return a LocalStatusRepository
@@ -84,4 +86,3 @@ class Repository(object):
         :param regtype: 'remote', 'local', or 'status'
         """
         pass
-