Support for offset commit and fetch requests available in Kafka 0.8.1

OffsetCommitRequest and OffsetFetchRequest are supported from Kafka 0.8.1. This pull request has added client API support for both using the methods offsetCommit and offsetFetch. The metadata parameter is part of the protocol, but not implemented in Kafka yet.

In addition, the pull request also includes a test-suite for the Client API, this requires a local Kafka server (tested with 0.8.1.1). Dependencies are updated to the latest versions of finagle, kafka and scalatest.