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.
`RandomRDD` should be sufficient to handle i.i.d. values. Since the only difference is the random number generator, it may not be worth subclassing.