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.
Hmm it's about a 50/50 split in the current implementations of Partition whether `with Serializable` is there. Removing as it is unnecessary. Can clean up some of the other ones in a separate patch.