Add support for file permissions

It should be straightforward to add  at least canned ACLs, after this line:
https://github.com/sbt/sbt-s3/blob/master/src/main/scala/S3Plugin.scala#L291.
http://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html#canned-acl

I could create a PR for this issue. Thoughts? 