Move chai and mocha to .devDependencies

Since chai is only used in testing, it's best to list it as a [dev dependency](https://npmjs.org/doc/json.html#devDependencies). Also, I made mocha listed as a dev dependency as well. Finally, instead of having to run `mocha test/spec` every time you want to test, you can instead use `npm test` (see [npm scripts](https://npmjs.org/doc/scripts.html))