Add AWS S3 deploy recipe

This adds a deploy recipe for an Amazon S3 hosted static site.

Run with `roots deploy --s3`. It requires an `s3.json` configuration file in the project's root directory with access key, secret, region, and bucket name.

One last feature I want to add (hopefully by tomorrow) is for the deployer to automatically create an Amazon S3 bucket from the project name if it's available and if a bucket name isn't specified in the config file. If the name is taken, it'll then ask the user to add an alternative name into the config file and will attempt to create a bucket with that name.

Mainly, submitting this for feedback and thoughts, I'm pretty new at open source contribution, would definitely love any advice.