Publish to NPM

I'd really like to see this project in NPM and took a pass at making some changes with that in mind. This PR;

* Adds package.json
* Adds tests
* Updates argument handling to use minimist (as optimist is no longer maintained)
* Switches AWS key/secret loading to use dotenv and matches environment variable names to what aws-cli uses
* Adds a `--query` argument to `dynamo-archive` to allow for exporting portions of tables.
besides, as I understand, your version of the script doesn't support `--key` and `--secret`, right? I don't think it's a good idea to remove this functionality entirely