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.
Cloneable is generally something you want to avoid extending as an interface since there's a host of issues with that interface. However, since we don't anticipate complex hierarchy and class members here, we can provide `def clone: RandomRDDGenerators = super.clone.asInstanceOf[RandomRDDGenerators]` in this trait.