For adding Kafka 0.9.0.0 support, this commit changes the following:
- Split up of the kafka connector into a flink-connector-kafka-(base|0.9|0.8) with different dependencies
- The base package contains common test cases, classes and implementations (the producer for 0.9 and 0.8 relies on exactly the same code)
- the 0.8 package contains a kafka connector implementation against the SimpleConsumer (low level) API of Kafka 0.8. There are some additional tests for the ZK offset committing
- The 0.9 package relies on the new Consumer API of Kafka 0.9.0.0
- Support for metrics for all producers and the 0.9 consumer through Flink's accumulators.

This closes #1489