Exports for Node.js and Browserify

Has backwards compatibility for the old `require()` API. If we're in the browser, export as a standard global object.
This is the way that [Underscore.js does it](https://github.com/jashkenas/underscore/blob/master/underscore.js#L41-L51) and as far as I know is the defacto standard. This is also where the ** around Node.js comes from.