Support for AWS S3 Standard - Infrequent Access Storage Class

Amazon recently announced a new S3 storage class, Infrequent Access: https://aws.amazon.com/blogs/aws/aws-storage-update-new-lower-cost-s3-storage-option-glacier-price-reduction/ . Its storage price is lower than Reduced Redundancy but has a higher per-access price tag, and storage cost is calculated with a minimum 30-day storage.

Duplicity has supported S3 Reduced Redundancy storage class. Let's add support for this new Infrequent Access storage class!