[SPARK-6919] [PYSPARK] Add asDict method to StatCounter

Add method to easily convert a StatCounter instance into a Python dict

https://issues.apache.org/jira/browse/SPARK-6919

Note: This is my original work and the existing Spark license applies.

Author: Erik Shilts <erik.shilts@opower.com>

Closes #5516 from eshilts/statcounter-asdict.