Implement support for Keyed producer

Provides support for two partitioners
* Round robin
* Hashed (default as per kafka clients)
Fix markup in README
Fix bugs and testing

* Ensure that round-robin partitioner works fine
* _load_metadata_for_topics() would cause duplicate and stale entries in
  self.topic_partitions. Fix this
Better examples in README
Merge pull request #32 from mahendra/keyed

Implement support for keyed messages