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)