Use HEAD /name to check bucket existence with S3

This method costs 0.4 cents per 10,000 requests instead of 0.5 cents
per 1,000 requests:

http://aws.amazon.com/s3/pricing/

API reference:

http://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketHEAD.html
I changed the signature, not the identity.  Refer to http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html for more information.