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.
This could only happen if we make RandomRDDGenerators a class that takes sc as a parameter. So if this is necessary, it's a reason for going with the class impl of RandomRDDGenerators.