browse images feature and new assets

I've implemented simple method (view, urlconf entry, widget options) to enable browsing of already uploaded images.

This feature requires sorl.thumbnail 

It also required me to update the source of Redactor.JS

in settings.py one can define the directories to be listed:

REDACTOR_BROWSE_FOLDERS = {'Default': REDACTOR_UPLOAD, 'OtherDir': '<path>'}