Warn against subclassing scala.App, and remove one instance of this in examples

Author: Sean Owen <sowen@cloudera.com>

Closes #3497 from srowen/SPARK-4170 and squashes the following commits:

4a6131f [Sean Owen] Restore multiline string formatting
a8ca895 [Sean Owen] Warn against subclassing scala.App, and remove one instance of this in examples