Add UMD Support for CommonJS

You can get around this by running this after requiring jQuery (from [Masonry Appendix](http://masonry.desandro.com/appendix.html#requirejs))

``` js
// require the require function
requirejs( [ 'require', 'jquery', 'path/to/masonry.pkgd.js' ],
  function( require, $, Masonry ) {
    // require jquery-bridget, it's included in masonry.pkgd.js
    require( [ 'jquery-bridget/jquery.bridget' ],
    function() {
      // make Masonry a jQuery plugin
      $.bridget( 'masonry', Masonry );
      // now you can use $().masonry()
      $('#container').masonry({...});
    }
  );
});
```

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would `module.exports = defineBridget( window.jQuery );` be the right way to go?