Add support of right join

[viirya]

It will be more convenient to have the support of right join.

[ankurdave]

Thanks! It's a good idea to support right outer joins. Could they be implemented in terms of left outer joins instead of as a separate primitive? Something like this:

def rightJoin[V2, V3](other: RDD[(K, V2)])(f: (K, Option[V], V2)) => V3): IndexedRDD[K, V3] =
  other.join(this) { (id, a, bOpt) => f(id, bOpt, a) }

[viirya]

Thanks for reviewing! I've updated this patch. The rightJoin in IndexedRDDPartition has been modified to use left joins. However, for another rightJoin that accepts other: Iterator[(K, V2)], because we can't also do other.leftJoin(this) here, I leave its implementation to PARTPartition.