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.
Calling it `Distribution` was actually Matei's suggestion at the initial design meeting. We all thought it's only going to need a nextDouble method before I actually started implementing things and realized more methods are needed.