Support for IMAPv4 ID extension

This patch enables the ID extension of IMAPv4.

The patch sends the client name and the client version to the server.

Usually, the server doesn't require it, but in some cases the server
drop de connection if the ID is not send.

#Close #71

offlineimap/imapserver.py

@@ -583,6 +583,11 @@ class IMAPServer:
                         af=self.af,
                     )
 
+                # If 'ID' extension is used by the server, we should use it
+                if 'ID' in imapobj.capabilities:
+                    l_str = '("name" "OfflineIMAP" "version" "{}")'.format(offlineimap.__version__)
+                    imapobj.id(l_str)
+
                 if not self.preauth_tunnel:
                     try:
                         self.__authn_helper(imapobj)