Make external API consistently support python3 strings for topic.

[kecaps]

I noticed that python3 string topics were supported in SimpleProducer.send_messages but not elsewhere in the API. This change makes python3 string topics supported throughout the public API.

This required changes in some testing code to ensure that bytes were used when building internal structs for return and sending to kafka service.

[dpkp]

why not simply set topic = kafka_bytestring(topic) ?

[kecaps]

I did it this way as a premature optimization because I saw has_metadata_for_topic and get_partition_ids_for_topic were on the callpath of send_messages which is probably the most used method and always calls these metadata methods with a byte string. But on closer inspection, it should only call them once on initialization, so its probably clearer to use a consistent pattern.

[dpkp]

thanks -- test error looks like a travis-ci failure. do you mind squashing your second commit or git commit --amend ?

[dpkp]

thanks!