Merge pull request #15 from merlinND/lighting

Area lights
Add Mersenne Twister library by Takuji Nishimura and Makoto Matsumoto

Adapted somewhat for C++.
Use Mersenne Twister instead of rand()
Pre-compute values in AreaLight
Various adjustments
Declare common colors as constants
Measure approximate execution time
Merge pull request #16 from merlinND/performance

Small performance improvements
Add lighting test scene
Refactor light participation computing
Basis for attenuation of lighting with distance
Attenuation function for punctual lights
Tweak test scene