Documentation: Materials browser

This isn't ready to be merged in, but I wanted to get some feedback before I put too much work into it. I'm adding some live scenes inside of the materials documentation so that people can play with live values to understand how things work a little bit better.

Live example: http://gregtatum.com/tmp/three.js/docs/#Reference/Materials/MeshPhongMaterial

The implementation isn't perfect yet with dat.gui as some of the values don't change things and end up crashing the preview if you do the wrong combo. It's mostly working, and so far seems to give a lot easier understanding about what is going on.

One thing I can't figure out is how to live-change the shading property. I'm also wondering if it's worth doing vertex colors, morphs, and some of the funkier things that require other object data to really pull off.