ScrambleSuit should make it easy for bridge admins to learn password

At this point, the password is stored in pt_state/scramblesuit/server_state.cpickle in Python's string encoding. However, bridge operators might want to test their bridge and we should make that easier.

Ideally, we would have a file descriptor.txt which contains a copy&pastable bridge descriptor including the password. That would also be consistent with how users learn about their hidden service descriptor.