SPARK-2626 [DOCS] Stop SparkContext in all examples

Call SparkContext.stop() in all examples (and touch up minor nearby code style issues while at it)

Author: Sean Owen <sowen@cloudera.com>

Closes #2575 from srowen/SPARK-2626 and squashes the following commits:

5b2baae [Sean Owen] Call SparkContext.stop() in all examples (and touch up minor nearby code style issues while at it)