Changes for aligning code with offset fetch and commit APIs (Kafka 0.8.1)

Commit # 5581649 ::
Kafka API keys have changed for offset fetch and commit requests.
Refer https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/api/RequestKeys.scala#L32

Commit # daabc9f ::
Latest code in the trunk no longer returns client_id in offset fetch and commit response.
Refer https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/api/OffsetFetchResponse.scala
and https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/api/OffsetCommitResponse.scala