Ensure proper cleanup of temporary directory

[jwodder]

The temporary directory used by S3Client for downloading manifests and inventory lists should be deleted automatically on program exit, but this is not happening (when Ctrl-C is pressed, at least). The tempfile docs suggest that File handles or similar might not be getting dropped before the TempDir. Fix this.

[jwodder]

Further investigation indicates that the temporary directory is cleaned up appropriately on normal (non-Ctrl-C) exit, so this is only a problem when Ctrl-C is hit, which we can't do much about at the moment (but see #14).