Implement Server SSL fingerprint check

If we connect to a SSL server (not STARTTLS) and no CA cert has been
specified for verification, we check the configured SSL fingerprint and
bail out in case it has not been set yet, or it does not match.

This means one more mandatory option for SSL configuration, but it
improves security a lot.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -182,6 +182,9 @@ class IMAPRepository(BaseRepository):
                                 % (self.name, cacertfile))
         return cacertfile
 
+    def get_ssl_fingerprint(self):
+        return self.getconf('cert_fingerprint', None)
+
     def getpreauthtunnel(self):
         return self.getconf('preauthtunnel', None)