Add SASL PLAIN authentication method

- this method isn't as deprecated as IMAP LOGIN;

 - it allows to keep hashed passwords on the server side;

 - it has the ability to specify that the remote identity
   is different from authenticating username, so it even
   can be useful in some cases (e.g., migrated mailboxes);
   configuration variable "remote_identity" was introduced
   to leverage this functionality.

From: Andreas Mack <andreas.mack@konsec.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/repository/IMAP.py

@@ -115,6 +115,18 @@ class IMAPRepository(BaseRepository):
                                    "'%s' specified." % self,
                                OfflineImapError.ERROR.REPO)
 
+
+    def get_remote_identity(self):
+        """
+        Remote identity is used for certain SASL mechanisms
+        (currently -- PLAIN) to inform server about the ID
+        we want to authorize as instead of our login name.
+
+        """
+
+        return self.getconf('remote_identity', default=None)
+
+
     def getuser(self):
         user = None
         localeval = self.localeval