Allow publishing to S3.

Sets up middleman-sync and adds the `middleman sync` command which will publish to an S3 bucket.

The AWS setting can be configured with the following ENV variables:

```ruby
  ENV['AWS_BUCKET']
  ENV['AWS_REGION'] || 'us-east-1'
  ENV['AWS_ACCESS_KEY']
  ENV['AWS_SECRET_ACCESS_KEY']
```

A few additional steps would be needed to switch the site over to S3 and/or CloudFront (namely DNS and initial S3 bucket setup).