This function returns a random integer from the negative binomial
distribution, the number of failures occurring before n successes
in independent trials with probability p of success. The
probability distribution for negative binomial variates is,
p(k) = {\Gamma(n + k) \over \Gamma(k+1) \Gamma(n) } p^n (1-p)^k
Note that n is not required to be an integer.