#13 Multithreaded pubsub

[schoenborno]

Extend pypubsub to support passing messages between threads. This could be done as per https://groups.google.com/forum/#!topic/pypubsub/p1n48gS4tMQ, or by creating a QueueingPublisher which would automatically subscribe its sendMessage() to another Publisher when subscribe() is called, automatically queue messages when sendMessage() is called, and provide an additional method dispatchQueuedMessages().