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* Updates argument handling to use minimist (as optimist is no longer maintained)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.
this looks like a code duplication to me. the code in these two functions is absolutely identical, except the name of the method on `dynamo`. would be much more convenient to pass the name as an argument to the function: `scan` or `query`. don't you think?