fix negbin.pdf

Hey, I noticed that the negbin.pdf algorithm was not behaving as expected, so I followed [this definition](http://www.wolframalpha.com/input/?i=negative+binomial+distribution&a=*C.negative+binomial+distribution-_*MathWorld-&f2=10&f=NegativeBinomialProbabilities.n%5Cu005f10&f3=0.25&f=NegativeBinomialProbabilities.p%5Cu005f0.25&f4=5&f=NegativeBinomialProbabilities.x%5Cu005f5&a=*FVarOpt.1-_***NegativeBinomialProbabilities.x--.***NegativeBinomialProbabilities.l-.*NegativeBinomialProbabilities.r---.*--) and picked up some mistakes, fixed them, and now it behaves accordingly to wolframalpha's implementation and microsoft excel's implementation (also added a test to confirm that).

Also added a test to assert the cdf.