Separate Consumer/Producer/Partitioner modules

No code changes, just splitting files into separate modules by function.

Consumers:
kafka.consumer.base
kafka.consumer.simple
kafka.consumer.multiprocess

Producers:
kafka.producer.base
kafka.producer.simple
kafka.producer.keyed

Partitioners:
kafka.partitioner.base
kafka.partitioner.roundrobin
kafka.partitioner.hashed