IMAP folder: expose the message keywords

The keywords are in the flag string, so imaputil can just strip the
usual \Flags.

Signed-off-by: Igor Almeida <igor.contato@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -420,6 +420,11 @@ class BaseFolder(object):
 
         raise NotImplementedError
 
+    def getmessagekeywords(self, uid):
+        """Returns the keywords for the specified message."""
+
+        raise NotImplementedError
+
     def savemessageflags(self, uid, flags):
         """Sets the specified message's flags to the given set.