[SPARK-11336] Add links to example codes

https://issues.apache.org/jira/browse/SPARK-11336

mengxr I add a hyperlink of Spark on Github and a hint of their existences in Spark code repo in each code example. I remove the config key for changing the example code dir, since we assume all examples  should be in spark/examples.

The hyperlink, though we cannot use it now, since the Spark v1.6.0 has not been released yet, can be used after the release. So it is not a problem.

I add some screen shots, so you can get an instant feeling.

<img width="949" alt="screen shot 2015-10-27 at 10 47 18 pm" src="https://cloud.githubusercontent.com/assets/2637239/10780634/bd20e072-7cfc-11e5-8960-def4fc62a8ea.png">

<img width="1144" alt="screen shot 2015-10-27 at 10 47 31 pm" src="https://cloud.githubusercontent.com/assets/2637239/10780636/c3f6e180-7cfc-11e5-80b2-233589f4a9a3.png">

Author: Xusen Yin <yinxusen@gmail.com>

Closes #9320 from yinxusen/SPARK-11336.