Adjust assets management to work with CDN tools like django-storages

[jeffjen]

Adjust assets management to work with CDN tools like django-storage

1. python manage.py collectstatic will upload content to with prefix json_widget
2. CSS and JS references CDN full name and path

[toracle]

I had similar problem and inspected into it but found a little simpler solution than yours.

css and js property of Media class is rendered by render_js and render_css method.

When look into that method, it calls absolute_path method to make a path absolute, and absolute_path calls static function, and it calls StaticNode.handle_simple method.

And, this handle_simple method do the same things you do, turn a path to absolute one with storage.url().

So, default django Media logic act like this:

1. If js or css path is an absolute path, use itself.
2. If they are relative one,
   1. If django.contrib.staticfiles app is installed, get a staticfile_storage which is defined by settings.STATICFILES_STORAGE value, and use storage.url(path) to convert it to absolute.
   2. If the app is not installed, just add STATIC_URL prefix to the path.

So, I think a simple solution here is that, just to remove any URL prefix and using a relative path.

[jmrivas86]

I close this PR because is solved in #30