Add dotenv gem to load credentials

### Changes

* Add the [`dotenv`](https://github.com/bkeepers/dotenv) gem to store the Amazon credentials
* Provide a `.env.sample` 

This removes the `export` usage step and allows the consumer to store their credentials in a `.env` file that will be loaded when `rake` is invoked.

### Usage

```sh
cp .env .env.sample
```

Fill out the `.env` file with your credentials (excluded from version control).