[SPARK-1308] Add partitions() method to PySpark RDDs

Per the discussion here:
http://apache-spark-user-list.1001560.n3.nabble.com/How-many-partitions-
is-my-RDD-split-into-td3072.html

Looking at the instructions here
(https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark
), it is not clear how to add tests for this kind of modification to
PySpark.

First-time committer. Forgive me if I’ve messed anything up.