fix(widget): ensure image is loaded before initializing widget

`jquery.ready` doesn't take into account the loading of images.
If the images are not loaded, we get wrong image dimension
causing the PPOI widget to fail.

**Suggestion**: @WGBH, we can also get rid of two different files, if we 
put the initialisation script in [js/versatileimagefield.js](https://github.com/WGBH/django-versatileimagefield/blob/master/versatileimagefield/static/versatileimagefield/js/versatileimagefield.js)
by appropriately selecting jquery, something on the line:

```
var jQuery = window.jQuery || django.jQuery;

if (jQuery){
     jQuery(document).ready(function(){
          jQuery('.sizedimage-preview').load(generateCenterpointWidget);
     });
}
```
Thoughts? I can sent you PR if you like. But wondering are there any specific
reason to have different files.