Fix S3 code to support all regions (and not just US Standard buckets)

I managed to write the S3 code to only work against the US Standard buckets.  I've fixed it to properly support all AWS regions.  

There is a change in the format of the S3 source urls that will cause users to have to update their usage.

Previously URLs were in the form:
    
    s3://bucket-name/filename.tar.gz

Now the URLs will need to include the S3 region endpoint:

    s3://s3.amazonaws.com/bucket-name/filename.tar.gz  # for a US Standard bucket 
    s3://s3-us-west-2.amazonaws.com/bucket-name/filename.tar.gz # for a bucket in US West (Oregon) region.