Get publishing-api working

In order to support publishing to both a draft and live environment we need an app to sit in front of the content-store(s) and route items accordingly.

This is the first step towards that. It passes content to the live content-store. For the time being it only allows configuring a single content-store.

Story: https://www.agileplannerapp.com/boards/173808/cards/8693
This has been resolved in 7352aca by returning the unmarshalled response from the url-arbiter. I chose this over return the raw response as we (the publishing-api app) can control this here.