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.
Right, except as written, normal returns a RDD[Double], which is a type that doesn't tell us anything about how the data was actually generated. Also, you're asking users to understand how to transform the normal distribution (which isn't hard but I wouldn't put it past them)