[SPARK-8444] [STREAMING] Adding Python streaming example for queueStream

A Python example similar to the existing one for Scala.

Author: Bryan Cutler <bjcutler@us.ibm.com>

Closes #6884 from BryanCutler/streaming-queueStream-example-8444 and squashes the following commits:

435ba7e [Bryan Cutler] [SPARK-8444] Fixed style checks, increased sleep time to show empty queue
257abb0 [Bryan Cutler] [SPARK-8444] Stop context gracefully, Removed unused import, Added description comment
376ef6e [Bryan Cutler] [SPARK-8444] Fixed bug causing DStream.pprint to append empty parenthesis to output instead of blank line
1ff5f8b [Bryan Cutler] [SPARK-8444] Adding Python streaming example for queue_stream