Support CommonJS module.exports.

I want to use this library on the server-side as well as the client-side. This PR contains logic to use module.exports if it’s available. It falls back to `window.NumberHelpers` so this should be compatible with the existing code today. (And the tests pass!)
This leading `@` is unnecessary since CoffeeScript will wrap your `class` in a closure and mount it to `this` anyway.