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.
Take some popular libraries as examples:

Matlab: http://www.mathworks.com/help/matlab/ref/randn.html
NumPy: http://docs.scipy.org/doc/numpy/reference/generated/numpy.random.randn.html

They are among the most called functions in numerical computation. Having less arguments defined in the method definitely has its merit. We need to support Java API, so we cannot reply on named arguments with default values (which is really convenient). In the doc, we should mention this is for standard normal distribution and give examples for shifting mean or scaling.