[SPARK-2014] Make PySpark store RDDs in MEMORY_ONLY_SER with compression by default

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #1051 from ScrapCodes/SPARK-2014/pyspark-cache and squashes the following commits:

f192df7 [Prashant Sharma] Code Review
2a2f43f [Prashant Sharma] [SPARK-2014] Make PySpark store RDDs in MEMORY_ONLY_SER with compression by default
Merge remote-tracking branch 'upstream/master' into decisiontree-api