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Setup 2-way AMQP and Invoca pub/sub simulators

The Broker will need to listen for AMQP messages as well as publishing
them. This is implemented for now using two separate queues, although
I'll need to read more about AMQP to see if there's a more semantic way.
I'm not wild about the queue names either.

Two scripts are added to `invoca/` to simulate sending/receiving AMQP
messages from the Invoca API end.