Gevent implementation of kafka-python

This adds a 'green' submodule to kafka-python, which uses the gevent python library (www.gevent.org) constructs for certain classes. The only classes I ported over right now are Producer, SimpleProducer, KeyedProducer, KafkaConnection and KafkaClient.

To use the submodule, simply do replace

import kafka

with 

import kafka.green as kafka

Comments welcome!