Add six to tox.ini testing deps
Merge pull request #223 from dpkp/metadata_refactor

Metadata Refactor

* add MetadataRequest and MetadataResponse namedtuples
* add TopicMetadata namedtuple
* add error codes to Topic and Partition Metadata
* add KafkaClient.send_metadata_request() method
* KafkaProtocol.decode_metadata_response changed to return a
  MetadataResponse object so that it is consistent with server api:
  [broker_list, topic_list]
* raise server exceptions in load_metadata_for_topics(*topics)
  unless topics is null (full refresh)
* Replace non-standard exceptions (LeaderUnavailable,
  PartitionUnavailable) with server standard exceptions
  (LeaderNotAvailableError, UnknownTopicOrPartitionError)

Conflicts:
	kafka/client.py
	test/test_client.py
	test/test_producer_integration.py
	test/test_protocol.py