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.
Transforming a Poisson distribution to another is not as easy as uniform or Gaussian. Thanks for reminding the range limit! It makes `RandomRDD` better than inline implementation. I don't think suclassing `RandomRDD` is necessary since we only generate data i.i.d. values.