Make AjaxImageField behave more like a standard django FileField

Hi, I like your lib - simple and uses django storage API. I thought you may want to include some of this... This basicly adds the FileDescriptor stuff from django.db.models.FileField (with the FileField.pre_save file saving excluded). With these changes I can use MyModel.image.url and rely on the storage backend to generate the correct media url or file path, removing the need for the PREPEND_MEDIA_URL setting. I also added a few tweaks like configurable AUTH_TEST, FILENAME_NORMALIZER.  I am using this lib with my own HashedFileStorage class - works great. Thanks :)