Merge pull request #13 from merlinND/geometry

Geometric plane
Add notion of "light participation" at a point
Fix circular dependency
Area light stubs
Auto normalize Ray's direction at construction
Area lighting algorithm
Use Monte Carlo sampling in area lighting
Add current render examples
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 Parallelepipoid (+ refactor Cube)

A Cube is a particular case of parallelepipoid
Add parallelepipoids to test scene