Add support for django-s3-storage-folder

Fixes a bug when using [django-s3-storage-folder](https://pypi.python.org/pypi/django-s3-folder-storage/), where filebrowser-safe will not set the appropriate StorageMixin because it doesn't recognize 's3_folder_storage.s3.DefaultStorage' as a valid value for DEFAULT_FILE_STORAGE.

Specifically, filebrowser-safe looks for filebrowser_safe.storage.DefaultStorageMixin, which doesn't exist. This leads to errors that look like:

```
AttributeError at /admin/storepage/storepage/131/
'DefaultStorage' object has no attribute 'isdir'
```

As a workaround, instead of defining a storage.DefaultStorageMixin class (which would have a misleading name, since it's not the default storage mixin), we just treat this as a special case and use the storage.S3BotoStorageMixin if we detect that django-s3-storage-folder is used.