add high level kafka consumer, integrate with zookeeper client to use group id for multiple clients.
code refactoring.
add native high level consumer, which directly get consumer offset from kafka instead of zookeeper; Need to add a check in the RefreshOffsets when offsets in record/zookeeper are out of the bound of actual logs.
check offset for consumer group if it's in a correct range; if not, correct it by setting it to minimum offset.
refactoring
Merge pull request #2 from aNutForAJarOfTuna/zookeeper_support

Zookeeper support without unit tests yet.