Preliminary work on materials
Add Checkerboard material
Add Material support to scene files

Not at all complete yet but the basic infrastructure is in place.
Named materials!
Disallow capital material names

They could posibly conflict with Material class names so lets just
side-step that problem.
Add scene support for Checkerboard
Update scenes to use Materials
Optimize shadow feelers

Merely checks for an intersection closer than the light distance
rather than finding the closest intersection and comparing the
distances.
Change parameters to const references

This provides marginal performance improvements.
Add refractive index to Material

Still need to actually use it, though.
Display ref{ra,le}ctive percentages
Calculate reflectance
Update ballsOnAPlane.scn to have refraction
First round of refraction

Totally doesn't work but it's a decent start to build off of.
Add refractiveIndex to Ray
Flip Sphere normal if inside of Sphere
Use castRay to do refractance rays

This meant including start and end materials in various places to
determine when to refract.
Update error text
Remove commented out code
Add refractive spheres to ballsOnAPlane