RightAws support for attachment_fu

Updated the :s3 backend to support use of RightAws for access to S3, alongside AWS::S3.

AWS::S3 is basically unmaintained now, whereas RightAws seems to be currently active.

If there's anything that I've done wrong or mucked up, I'd be happy to fix it -- thanks!

This is an updated pull request with a few more commits that help fix bugs and tweak the behaviour of the RightAws backend.