Make producers take a topic argument at send rather than init time
This allows a single producer to be used to send to multiple topics.
See https://github.com/mumrah/kafka-python/issues/110
Use TopicAndPartition when producing async messages
Merge pull request #111 from rdiomar/multitopic_producers

Make producers take a topic argument at send rather than init time -- fixes Issue #110, but breaks backwards compatibility with previous Producer interface.