Hello, I noticed that the README suggests starting a separate thread to call listen(), but I don't believe the underlying socket is thread-safe (unless I'm missing something of course).

I believe I've worked out an implementation that at least allows send_message() to avoid clobbering any enquire_link or other PDUs the socket might be exchanging at the time send_message() is called.

Would it be worth cleaning this up and submitting a PR? If so, how would you like me to approach it? A few options I can think of:

1. Include the minimal hooks necessary (if any) for someone to implement their own client outside of smpplib proper
2. Update the existing client to use the thread-safe socket implementation
3. Include an alternate ThreadSafeClient implementation, as in the linked gist

Thanks for this library and in advance for any input you may have!