Performance optimizations needed to make S3_website diff operation much faster

Summary:
* Improve performance
* Calculate MD5 of gzipped S3 object using its gzipped content (previously MD5 was always calculated using raw, non-gzipped file content).

It's mostly related to S3_website performance issues discussed here: https://github.com/laurilehmijoki/s3_website/issues/44

I bumped version to 2.0.0, as the new way of MD5 calculation for gzipped files can be treated as non-backward compatible change.