SPARK-1446: Spark examples should not do a System.exit

Spark examples should exit nice using SparkContext.stop() method, rather than System.exit
System.exit can cause issues like in SPARK-1407

Author: Sandeep <sandeep@techaddict.me>

Closes #370 from techaddict/1446 and squashes the following commits:

e9234cf [Sandeep] SPARK-1446: Spark examples should not do a System.exit Spark examples should exit nice using SparkContext.stop() method, rather than System.exit System.exit can cause issues like in SPARK-1407
Revert "SPARK-729:  Closures not always serialized at capture time"

This reverts commit 8ca3b2bc90a63b23a03f339e390174cd7a672b40.
Merge branch 'master' of https://github.com/apache/spark into serviceLifeCycle