[Code review only] Add counter&aggregator for kafka input operator

Just for code review, I'll create another pull request for this

Counter result looks like this:
http://node0.morado.com:9090/ws/v1/applications/application_1411527433210_0690/physicalPlan/operators/1

Aggregator result looks like this:
http://node0.morado.com:9090/ws/v1/applications/application_1411527433210_0690/logicalPlan/operators/KafkaBenchmarkConsumer/aggregation