A simpler kafka consumer

```
kafka = KafkaConsumer('topic1')
for m in kafka:
    print m

kafka = KafkaConsumer('topic1', 'topic2',
                      group_id='my_consumer_group',
                      auto_commit_enable=True,
                      auto_commit_interval_ms=30 * 1000,
                      auto_offset_reset='smallest')

for m in kafka:
    process_message(m)
    kafka.task_done(m)
```
This logic makes me cry.  Isn't this equivalent to:

    if self._config[config_key] != None and not isinstance(self._config[config_key], six.binary_type):
        self._config[config_key] = self._config[config_key].encode('utf-8')

This generally assumes that we're looking at a unicode/string object?  Wouldn't this be a better check:

    if isinstance(self._config[config_key], six.string_types):
        self._config[config_key] = self._config[config_key].encode("utf-8")

?