Add cool sky

[xanxys]

Preetham model seems good enough for daylight.

[xanxys]

Papaers:

• Preetham http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
• Nishita http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.75.5595&rep=rep1&type=pdf

Good sites:

• http://www.scratchapixel.com/lessons/3d-advanced-lessons/simulating-the-colors-of-the-sky/atmospheric-scattering/
• http://www.geo.mtu.edu/~scarn/teaching/GE4250/scattering_lecture_slides.pdf