Run webpack dev server and express in tandem

![screen shot 2014-10-17 at 10 17 57 am](https://cloud.githubusercontent.com/assets/590904/4680143/6cf1b020-5608-11e4-82d6-ecaea99c9253.png)

I'm going to rename this `asset_host` because @dce showed me http://api.rubyonrails.org/classes/ActionView/Helpers/AssetUrlHelper.html