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.
Actually, if I think about the most common use case, generator probably won't be called more than once (since the user would probably only be interested in a particular distribution for the RDD), in which case the factory methods are probably more desirable (one less line of code)