Duplicate key support

[sgentle]

For some reason, VDF/KeyValue supports duplicate keys. I added support to turn them into arrays (you can see an example in the updated test.js)

I also fixed up the package.json a little. npm start doesn't do anything for a library, util is a stdlib package (included with node) so we don't need it in the dependencies, and if you include nodeunit in your devDependencies then you can run the tests with npm test.