[WIP] Fixed MU Importer

This is an early PR done with the hope of getting one or two pairs of eyes on what I'm doing.

The changes I've made should be working, but I intend to further clean the code and write several more test cases covering bad data, and what happens when importing on top of existing data.

Please do [pull this down](http://dev.ghost.org/easy-git-pr-test/) and try importing data which references multiple users.