Make require() compatible with using the engine as a RequireJS package

Hi whirlibulf,

Thanks for your engine, I can't wait to give it a test drive! I was having problems loading it in AMD style with RequireJS and I found out it was because of the `.js` in statements like `require("./foo.js")`.

The modules still load properly without the .js extension. I ran the tests and it's fine (except of course the test `expect(false).toBe(true);`).

More info: http://requirejs.org/docs/api.html#jsfiles
> If a module ID has one of the following characteristics, the ID will not be passed through the "baseUrl + paths" configuration, and just be treated like a regular URL that is relative to the document:
> * Ends in ".js".
> * Starts with a "/".
> * Contains an URL protocol, like "http:" or "https:".

Thanks!