Increased light2 strength in triplet scene
Add default skybox
Fix makefile detecting already built files

Make would build every object file every time it was invoked. Fixed the issue so
now make only builds what is necessary.
Sphere inherits from Object, Color class

Add a color class to hold color information, and a base object class to define an
interface for renderable objects. Sphere inherits from object and is responsible
for its own intersection calculation.
Restructure project into subdirectories

Add subdirectory libraries math, render and scene to better organize the code.