Properties
Based on "A Practical Analytic Model for Daylight" aka The Preetham Model, the de facto standard analytic skydome model http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
First implemented by Simon Wallner http://www.simonwallner.at/projects/atmospheric-scattering
Improved by Martin Upitis http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
Three.js integration by zz85 http://twitter.com/blurspline
Methods
_registerExprSymbols()
Add the pre-defined standard functions to the ExprManager.