[SPARK-1022][Streaming] Add Kafka real unit test

This PR is a updated version of (https://github.com/apache/spark/pull/557) to actually test sending and receiving data through Kafka, and fix previous flaky issues.

@tdas, would you mind reviewing this PR? Thanks a lot.  
Its better to move all these utility functions for generating local kafka test harness to a separate class, say KafkaTestUtils.