add deployment to Amazon s3

* use the ruby-s3cmd gem and its sync command

The s3 configuration is controlled by the profiles' deploy section which must have `type: s3` and a `bucket` corresponding to the S3 bucket to deploy to.
* For example

    profiles:
      staging:
        deploy:
          type: s3
          bucket: s3://staging.jmesnil.net/
      production:
        deploy:
          type: s3
          bucket: s3://jmesnil.net/