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.
It just seems weird to break `numDataPoints` and `numPartitions` apart with `mean` in the middle, and I don't think `mean` should go before `numDataPoints` (renamed to `size`)