Implement traceur.require

This adds a `require` method to the `traceur` object (in Node.js only) which allows you to require js files that go through traceur just like you would require any other js file in Node.js.

```js
var traceur = require('./src/node/tracuer.js');
var x = traceur.require('./x.js');
```

or

```js
require = require('./src/node/tracuer.js').require;
var x = require('./x.js');
```

Fixes #239