use node.js style require

This makes verlet-js publishable on npm,

npm makes managing modules way way easier,
and works well on the client if you bundle modules with [browserify](https://github.com/substack/node-browserify)

I also have a branch that I've ported to use [vec2](https://npm.im/vec2),
I wanted to write a spatial hash lib, to use to efficiently create and destroy
constraints to nearby nodes, but I wanted it to work with a generic,
community supported vector lib - because then it would be usable with other people's modules.

However, this pull request is just about getting verlet into npm!