Updates to Webpack

Reconfigures ddm-dropdown to use webpack to compile all the assets used
by dropdown into a single JavaScript file. This includes the necessary
CSS. It also allows us to distribute the file in a UMD format.

Changes build process from gulp to webpack where possible.

Splits dropdown.js into three separate files in a components folder
that uses require to bring in necessary dependencies.
You can use `loaders`:

```javascript
  module: {
    loaders: [
      { test: /\.jsx$/, loader: 'jsx-loader?harmony' },
      {
        test: '/\.scss$/', loaders: [
          'style-loader',
          'css-loader',
          'autoprefixer-loader?{browsers:["last 2 version", "> 1%", "ie 8"]}',
          'sass-loader'
        ]
      }
    ]
  },
```

A little nicer than carefully placing `!` and using `.join()` yeah?

See http://webpack.github.io/docs/configuration.html#module-loaders

I've already made that change on my side to try it before telling you to use it. I can commit if you want.