Add `webpack-dev-server` to `devDependencies`
Add `dev` npm script to run `webpack-dev-server`

Now `npm run dev` runs `webpack-dev-server` with the desired options.

Want to put all the configuration inside the `devServer` object of
`webpack.config.js`, but `outputFile` didn't seem to work there, so
used the `--output-file` command-line flag instead. Left a question
in https://gitter.im/webpack/webpack about this.
Merge branch 'dev'