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.