Work-in-progress attempt at a producer test case.

Doesn't fully work yet, but getting close.
Merge branch 'master' into kafka-async-error
Working demo of Async producer behaviour.

This test illustrates the behaviour of the Asynchronous Kafka producer.
Specifically, that if a "kafka.common.MessageSizeTooLargeException" error
is encountered, the entire batch of messages will be lost.
Merge pull request #20 from mozilla-metrics/kafka-async-error

Add a test case to show the Async producer behaviour