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.
Having `nextDouble` and `setSeed` is essentially a pseudorandom number generator. Since if we want to expose `randomRDD` and `randomVectorRDD`, we have to expose this interface. Calling it `RandomNumberGenerator` sounds better than `Distribution` to me.