(feature) Add Amazon S3 backend

[biw]

This PR adds Amazon S3 backend support to Nuts. Assets are proxied through Nuts and do not need to be public. Access to the assets is facilitated through Amazon access key and secret key which can be set through environment variables. This is a rudimentary implementation and serves as a base.

Added Environment Variables

* `NUTS_BACKEND`: Select backend to use. (`github`, `s3`)

* `AWS_ACCESS_KEY_ID`: Your AWS Access Key ID which can access the assets

* `AWS_SECRET_ACCESS_KEY`: Your AWS Secret Access Key

* `AWS_BUCKET`: The Amazon S3 bucket containing the assets (Ex: `my-bucket`)

* `AWS_RELEASES_PREFIX`: An optional path prefix to the releases. (Ex: `my-app/releases`)

From GitbookIO/nuts#105