Refactoring Signatures to allow generic v4 presigned URLs

- Deprecating S3SignatureInterface
- Adding createPresignedUrl to SignatureInterface
Updating Amazon EC2 to latest version
Updating ElasticBeanstalk model to latest version
Updating OpsWorksClient to the latest version
Adding support for CopySnapshot presigned URLs

- Automatically computing a PresignedUrl value when copying snapshots
- Adding back the DescribeVpcPeeringConnections iterator
- Not adding Content-MD5 to the query string when signing URLs
2.6.5 release