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.
I didn't get exactly what you meant. I'm proposing the following:

~~~
val rdd = RandomRDDs.normal(n, p, seed).map(v => mean + sigma * v)
~~