Some assorted fixes for the SFTP backend.

Some assorted fixes for the SFTP backend.

  - Automatically call the close() method of a storage backend when the
    object is garbage collected.
  - Calling stat in the SFTP backend when the file doesn't exist will raise
    a Cumulus NotFoundError instead of a generic IOError.
  - Avoid the use of keyword arguments when calling SFTPClient methods (in
    my testing the first argument might be called 'path' instead of
    'filename', but avoid the problem altogether by just using positional
    arguments).