Added $script.done to enable the explicit trigger of an id being ready. 

This addition adds a much needed ability with very little code added.

Here is an example of what you can do using the done function:

my-awesome-plugin.js :
```javascript
$script.ready('jquery', function() {

  //define awesome jquery plugin here

  $script.done('my-awesome-plugin');
});
```

index.html :
```html
<script>
$script('jquery.js', 'jquery');
$script('my-awesome-plugin.js');
$script.ready('my-awesome-plugin', function() {
  //use my awesome plugin here
});
</script>
```