Blender Git Loki

Updated color calculus for sunsky

- removed ugly clamping function (it was dividing XYZ based on max of
one of the values)
- added option to use Exposure, this only works for brightness (Y).
results look very pleasant, foggy and hazy results are possible.
with exposre==0, no exposure happens for HDR extreme range skies,
this is how yafray rendered it.
- added menu for choosing color spaces (CIE = modern lcds)

Please review! (and yes i know it's still not in World :)