Updating the AmazonS3 class to support AWS Signature version 4 because
version 2 is no longer supported in all AWS regions. The version can be
selected with the new 'aws.api.signature.version' property (defaults to
2 for backwards compatibility). When set to '4', the user must also
specify the AWS region via the 'region' property. The 'region' property
must match the region that the 'domain' property resolves to.
Bug: 579907
Change-Id: If289dbc6d0f57323cfeaac2624c4eb5028f78d13