Major API changes and breakage.
- ListObjectsV2 implementing new ListObjects V2 S3 API.
- Transparent retry logic.
- GetObject() fixes an optimization.
- GetBucketPolicy and SetBucketPolicy APIs
- Presigned operations return *url.URL
- FPutObject auto-detect mime-type
- Support for S3 China region
- Payload checksum for signature is not calculated if the end point is https