Merge @joekiller's Kafka plugin to logstash-core

Closes #1472
This setting does a little magic so I think it should be noted:

> If reset_beginning is true, the consumer will check ZooKeeper to see if any other group members are present and active. If not, the consumer deletes any offset information in the ZooKeeper and starts at the smallest offset. If other group members are present, reset_beginning will not work and the consumer will rejoin the consumer group.