RandomRDDGenerator: initial design

Looking for feedback on design decisions. Very rough draft and untested. Please ignore style issues. I would like suggestions on better naming though.

RandomVectorRDD would be very similar to RandomRDD, but emit an Iterator of Vectors instead.
This mapping logic is probably better handled if we can have a NormalRandomRDD class. FactoryName.mapNormalRDD(rdd:RDD[Double]) seems unsafe and awkward.