Allowing expiry of log messages in RabbitMQ

[gregbaker]

This adds an optional expiration argument to the Handlers, which is passed to the pika BasicProperties object.

This would allow users to have log messages expire, in the case that some consumer created a queue that was subscribed to the topic, but is no longer consuming those messages (e.g. a debugging tool that only consumes log messages sporadically). e.g. this would create a handler whose log messages expire after 5 seconds:

rabbit = RabbitMQHandler(expiration='5000')

(Taking a string to represent an integer is quirky, but that mirrors the BasicProperties constructor.)