[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.

Author: jerryshao <saisai.shao@intel.com>

Closes #1751 from jerryshao/kafka-unit-test and squashes the following commits:

b6a505f [jerryshao] code refactor according to comments
5222330 [jerryshao] Change JavaKafkaStreamSuite to better test it
5525f10 [jerryshao] Fix flaky issue of Kafka real unit test
4559310 [jerryshao] Minor changes for Kafka unit test
860f649 [jerryshao] Minor style changes, and tests ignored due to flakiness
796d4ca [jerryshao] Add real Kafka streaming test