Skin shader for Unreal

Maybe the most research intensive shader i have ever made, full of workarounds to fake how light goes behind the skin and scatters.
Common methods are based on screen-space diffuse light blur. Since this is not possible in unreal, i had to find the way to get a similar result. The most common alternative are wrapped NdotLs to create layers wich are blended depending on the light vector.
The last change I have included has been a more physically accurate reflection model.
The BRDF used is the Kelemen Szirmay-Kalos, as proposed by NVIDIA.