Using the Right AWS gem for better fault tolerance

# Changes

* Switches from `aws-s3` gem to `right_aws` as it solves connection reset issues with automatic retry features
* Removes splitting feature for backup files, as `right_aws` can upload files exceeding 2 GB
* Returns exit code of zero when all files have been uploaded (previously returned 1; this didn’t appear to reflect failure)
* Unexpected exceptions will be raised; I haven’t had enough test time to find edge cases

# Testing

* I ran this against Magento 1.12 on Ubuntu 12.04 LTS with a MySQL 5.6.11 back end
* No changes are destructive or risky