Cycles / Sky Model:

* Replaced the Preetham model with the newer Hosek / Wilkie model:
"An Analytic Model for Full Spectral Sky-Dome Radiance" http://cgg.mff.cuni.cz/projects/SkylightModelling/ 

* We use the sample code data, which comes with the paper, but removed some unnecessary parts, we only need the xyz version.
* New "Albedo" UI paraemeter, to control the ground albedo (between 0 and 1). 
* Works with SVM only atm (CPU and CUDA). 

Example render:
http://www.pasteall.org/pic/show.php?id=57635

ToDo / Open Questions:
* OSL still uses the old model, will be done later. In the meantime it's useful to compare the two models this way.
* The new model needs a much weaker Strength value (0.01), otherwise it's white. Can this be fixed? 
* Code cleanup.

intern/cycles/render/nodes.h

@@ -107,6 +107,7 @@ public:
 
 	float3 sun_direction;
 	float turbidity;
+	float albedo;
 };
 
 class OutputNode : public ShaderNode {