Enhanced security for private S3 repos

[danklynn]

• Defaulted bucket permissions to CannedAccessControlList.AuthenticatedRead.
• Added support for S3 server-side encryption. On connect, S3Wagon attempts to detect if your bucket requires SSE by uploading a file without requesting SSE. If this request fails, the we assume that the policy requires SSE and use it for all future PutObjectRequests.