Returns the natural logarithm of the Gamma function evaluated at `x.' Mathematically, the Gamma function is defined to be the integral from 0 to Infinity of t^x exp(-t) dt. The implementation is copied, with extensions for the reflection formula, from Numerical Recipes in C, section 6.1. The argument `x' must be positive. Full accuracy is obtained for x>1. For x<1, the reflection formula is used. The computation is done using double-floats, and the result is a double-float.