Feature: Proxy Configuration

We're using the proxy options on the webpack-dev-server to redirect calls to various APIs when developing locally.

I've essentially copied over the code that webpack-dev-server uses (see https://github.com/webpack/webpack-dev-server/blob/master/lib/Server.js) to initialise the proxy using http-proxy-middleware (see https://github.com/chimurai/http-proxy-middleware), so that spinning up webpack-dev-server via this module when running tests in WebDriver also initialises the right proxy settings if required.