Read S3 bucket from SQS msgs

This is a companion PR to https://github.com/turbovote/s3-ftp/pull/2. That PR changes the format of the SQS messages from s3-ftp, so this PR updates usps-processor to read them in the new format.

It also standardizes the SQS config a bit by allowing the use of short queue names rather than the full URLs (since the SQS library can easily derive those for us).