This function returns a random integer from the hypergeometric
distribution. The probability distribution for hypergeometric
random variates is,
p(k) = C(n_1,k) C(n_2, t-k) / C(n_1 + n_2,k)
where C(a,b) = a!/(b!(a-b)!). The domain of k is
max(0,t-n_2), ..., max(t,n_1).