Added support for custom REDACTOR_FILE_STORAGE value

For my application, I needed to be able to specify a unique file storage other than the default storage in my settings file.

This will default to the current `default_storage` class previously used, but will let some users add a dot-path to the file storage class they want to use instead. This is done using the new `REDACTOR_FILE_STORAGE` value in `settings.py`.

```
# settings.py

# ...

REDACTOR_OPTIONS = {'lang': 'en'}
REDACTOR_UPLOAD = 'redactor/'
REDACTOR_UPLOAD_HANDLER = 'redactor.handlers.UUIDUploader'
REDACTOR_FILE_STORAGE = 'app.fs_container'  # defaults to 'django.core.files.storage.default_storage'
```

Links for writing custom file storage class (e.g. for using cloud object storage APIs instead of local storage):
https://docs.djangoproject.com/en/dev/howto/custom-file-storage/
https://docs.djangoproject.com/en/dev/ref/files/storage/