[SPARK-5946] [STREAMING] Add Python API for direct Kafka stream

Currently only added `createDirectStream` API, I'm not sure if `createRDD` is also needed, since some Java object needs to be wrapped in Python. Please help to review, thanks a lot.

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

Closes #4723 from jerryshao/direct-kafka-python-api and squashes the following commits:

a1fe97c [jerryshao] Fix rebase issue
eebf333 [jerryshao] Address the comments
da40f4e [jerryshao] Fix Python 2.6 Syntax error issue
5c0ee85 [jerryshao] Style fix
4aeac18 [jerryshao] Fix bug in example code
7146d86 [jerryshao] Add unit test
bf3bdd6 [jerryshao] Add more APIs and address the comments
f5b3801 [jerryshao] Small style fix
8641835 [Saisai Shao] Rebase and update the code
589c05b [Saisai Shao] Fix the style
d6fcb6a [Saisai Shao] Address the comments
dfda902 [Saisai Shao] Style fix
0f7d168 [Saisai Shao] Add the doc and fix some style issues
67e6880 [Saisai Shao] Fix test bug
917b0db [Saisai Shao] Add Python createRDD API for Kakfa direct stream
c3fc11d [jerryshao] Modify the docs
2c00936 [Saisai Shao] address the comments
3360f44 [jerryshao] Fix code style
e0e0f0d [jerryshao] Code clean and bug fix
338c41f [Saisai Shao] Add python API and example for direct kafka stream