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fix PRIVATE_STORAGE_AUTH_FUNCTION path, fixes #6

Fix import reverse

Update documentation on sending files, given that ``wsgi.file_handler`` support lives in Djagno 1.8+

Supports:
- AWS_PRIVATE_S3_ACCESS_KEY_ID / AWS_PRIVATE_ACCESS_KEY_ID / AWS_S3_ACCESS_KEY_ID / AWS_ACCESS_KEY_ID
- AWS_PRIVATE_S3_SECRET_ACCESS_KEY / AWS_PRIVATE_SECRET_ACCESS_KEY / AWS_S3_SECRET_ACCESS_KEY / AWS_SECRET_ACCESS_KEY

This is needed to fetch content from S3 without needing a full_path

This class can now be selected with the new `PRIVATE_STORAGE_CLASS` setting.

The default `private_storage.storage.PrivateStorage` class now points to
the class that was imported through the new setting, theirby preserving
backwards compatibility.

- Added get_path()
- Added get_private_file()
- no longer depend on `self.kwargs['path`]`.