Add CommonJS support

Simple change to allow CommonJS users to require Shuffle as a module:

```javascript
var Shuffle = require('Shuffle');

jQuery(function($) {
  $('.posts').each(function() {
    var itemSelector = '.post',
        shuffle = new Shuffle(this, {
          gutterWidth: 0,
          itemSelector: itemSelector,
          sizer: $(this).find(itemSelector).first()
        });
  });
});
```

I only had to add two lines to the [intro.js](https://github.com/thirdender/Shuffle/blob/71b2b7f7229088e0f57cdf2534eba6e4f8d7abeb/src/intro.js) file, and change the path of the `main` property in [package.json](https://github.com/thirdender/Shuffle/blob/71b2b7f7229088e0f57cdf2534eba6e4f8d7abeb/package.json).