[SPARK-6211][Streaming] Add Python Kafka API unit test

Refactor the Kafka unit test and add Python API support. CC tdas davies please help to review, thanks a lot.

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

Closes #4961 from jerryshao/SPARK-6211 and squashes the following commits:

ee4b919 [jerryshao] Fixed newly merged issue
82c756e [jerryshao] Address the comments
92912d1 [jerryshao] Address the commits
0708bb1 [jerryshao] Fix rebase issue
40b47a3 [Saisai Shao] Style fix
f889657 [Saisai Shao] Update the code according
8a2f3e2 [jerryshao] Address the issues
0f1b7ce [jerryshao] Still fix the bug
61a04f0 [jerryshao] Fix bugs and address the issues
64d9877 [jerryshao] Fix rebase bugs
8ad442f [jerryshao] Add kafka-assembly in run-tests
6020b00 [jerryshao] Add more debug info in Shell
8102d6e [jerryshao] Fix bug in Jenkins test
fde1213 [jerryshao] Code style changes
5536f95 [jerryshao] Refactor the Kafka unit test and add Python Kafka unittest support