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)
```
It looks like it's not a part of the high level consumer design.  I think it should be, for us.  The most obvious solution would be to pass in a dict of offsets, but it might be easier for other languages [Java] to add a method for directly updating the offset of a given partition.

    def update_offset(self, topic, partition, new_offset):
        ...