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

Commit # 2c15fae ::
Refer https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/api/RequestKeys.scala#L32

Commit # 5154afe ::
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

Commit # 8bf10d5 ::
Fixes the problem where consumer keeps on reading messages starting from last committed offset.