Merge pull request #74 from intel-hadoop/master

sync with master
Merge pull request #75 from clockfly/master

task recovery
Merge pull request #77 from clockfly/master

fix recovery bug
load kafka configuration only once
add Grouper, KafkaConsumer and fix configuration bugs
add license header
Merge pull request #80 from manuzhang/at_least_once

add multi-topic support to KafkaSpout
periodically checkpoint messages and timestamp to Kafka
Merge pull request #81 from manuzhang/at_least_once

periodically checkpoint messages and timestamp to Kafka
start consuming from checkpoints
Merge branch 'at_least_once' of https://github.com/manuzhang/gearpump into at_least_once
move throughput metrics to KafkaBolt
separate Checkpoint api and impl from examples
Merge remote branch 'upstream/master' into at_least_once
fix checkpoint bugs and add logs
Merge pull request #88 from manuzhang/at_least_once

add checkpointing