Update demo to load three.js as an NPM module

Moves the demo to its own directory (to avoid a `devDependency` on voxel-walk, which itself has minecraft-skin as a `dependency`), and changes it to depend on three.js from NPM instead of a script src in html. This includes an update from three.js r55 to r58 (no code changes required), since >r58 incompatible (separate issue, GH-5)