Add an interface to zip iterators of multiple RDDs
The current code supports 2, 3 or 4 arguments but can be extended
to more arguments if required.
Rename to zipPartitions and style changes
Rename classes from MapZipped* to Zipped*
Actually rename classes to ZippedPartitions*
(the previous commit only renamed the file)
Rename partition class and add scala doc
Merge pull request #582 from shivaram/master

Add zip partitions interface