add s3 deployment

* add deployment to Amazon s3 using ruby-s3cmd gem
* the S3 configuration is controlled by the profiles' deploy section which must have `type: s3` and a `url` corresponding to the S3 bucket to deploy to.

```yaml
  profiles:
    staging:
      deploy:
        type: s3
        url: s3://staging.jmesnil.net/
    production:
      deploy:
        type: s3
        url: s3://jmesnil.net/
```

* The deployment to s3 is using the sync command