Gamma ratio and delta ratio

The recently ported gamma functions from the Boost C++ library contain a method for computing a ratio of gamma functions:

gamma_ratio(a, b) = gamma(a) / gamma(b)

gamma_delta_ratio(a, delta) = gamma(a) / gamma(a + delta)

This method is required for part of the Boost C++ implementation of the RegularizedBeta function.

It has an application in Commons Statistics for computing the mean and variance of the Nakagami distribution.

Potential API:

// Added to the existing Gamma class
Gamma.ratio(a, b)
Gamma.delta_ratio(a, delta)

// New class
GammaRatio.value(a, b)
GammaRatio.delta(a, delta)