add package.json for publishing to npmjs.org

Issue #49.
Simply pull this and do `npm publish` (you'll have to create an account).

If you would like to create your own package.json, use `npm init` when in root. Entry point (default index.js) is not really needed, as this is not a node package - see [What is a module?](https://www.npmjs.org/doc/misc/npm-faq.html). As this is set default, I have removed it from package.json after `npm init`.

Quoting the relevant part:
*Most npm packages are modules, because they are libraries that you load with require. However, there's no requirement that an npm package be a module! Some only contain an executable command-line interface, and don't provide a main field for use in Node programs.*