Add UMD Support for CommonJS

Oh I see so jquery-bridget is actually a devDependency for outlayer, not a runtime dependency and therefore isn't actually needed to give masonry CommonJS support, but is needed to make masonry a jquery plugin.

The problem with `module.exports = defineBridget( window.jQuery );` is that in order use it in a CommonJS / browserify environment we you would have add jQuery to the root scope, which could get messy. 

What if instead of that we did `module.exports = defineBridget;`

Then we could avoid adding jQuery to the root scope and it could hypothetically be used in a CommonJS / browserify environment like this:

```
var $ = require('jquery');
var Masonry = require('masonry');
require('jquery-bridget')($);
$.bridget( 'masonry', Masonry );
$('#container').masonry({...});
```