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

I've added the `partitions()` method 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.

`sbt/sbt test` has 1 test fail -- `org.apache.spark.sql.hive.execution.HiveCompatibilitySuite` -- but this appears to be unrelated to my change. Reverting my changes and re-reunning `assembly` and `test` yield the same result.

First-time committer. Forgive me if I’ve messed anything up.