make changes to be more fault tolerant: clean up connections, brokers, failed_messages

- add integration tests for sync producer
- add integration tests for async producer w. leadership election
- use log.exception
Merge pull request #55 from quixey/fault-tolerance

Improve fault tolerance by handling leadership election and other metadata changes

Thanks, @jimjh!
Merge branch 'master' into prod-windows

Conflicts:
	kafka/producer.py