Kafka support

[briehl]

Start the Kafka listener with make start_kafka_listener.

Still needs tests, but I want to see them pass on Travis as-is, with the config changes.

[coveralls]

Pull Request Test Coverage Report for Build 227

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 89.903%

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Totals
Change from base Build 217:	-0.5%
Covered Lines:	1576
Relevant Lines:	1753

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💛 - Coveralls

[briehl]

This works when tested locally with a local Kafka install, but it doesn't like testing on Travis-CI. Not sure why. Either way, this work isn't critical for the GSP, so I propose we wait a bit.