Add collectPartition to JavaRDD interface.
Also remove takePartition from PythonRDD and use collectPartition in rdd.py.
Make collectPartitions take an array of partitions
Change the implementation to use runJob instead of PartitionPruningRDD.
Also update the unit tests and the python take implementation
to use the new interface.
Add comment explaining collectPartitions's use
Merge pull request #276 from shivaram/collectPartition

Add collectPartition to JavaRDD interface.

This interface is useful for implementing `take` from other language frontends where the data is serialized. Also remove `takePartition` from PythonRDD and use `collectPartition` in rdd.py.

Thanks @concretevitamin for the original change and tests.