replaced custom impl of Gamma fn with a lib, #48

github.com/substack/gamma
expanded supported range of the Beta fn, #48

Doing
    Β(n,k) = exp(lnΓ(n) - lnΓ(k) - lnΓ(n-k))
instead of
    B(n,k) = Γ(n) / (Γ(k) Γ(n-k))