Enable dist files to work with CommonJS

These three simple lines enable the client-side `dist` files to work with a CommonJS library like Stitch or Browserify.  With a little trick in bundling, you can now do this in the browser:

```js
var Handlebars = require('handlebars');
```

This becomes increasingly important for tools like [Rendr](https://github.com/airbnb/rendr) that allow running code isomorphically on the client and server.