See diff with previous editions

This allows users to view the changes between consecutive editions:

![screen shot 2014-08-29 at 12 33 28](https://cloud.githubusercontent.com/assets/3687/4089673/ab5ad134-2f70-11e4-8c15-25803d94fe50.png)

It also updates the "history & notes" tab to list the version history consistently with the way publisher does:

![screen shot 2014-08-29 at 12 36 40](https://cloud.githubusercontent.com/assets/3687/4089678/dab3c0bc-2f70-11e4-96f0-396bbb96a40c.png)

Story: https://www.agileplannerapp.com/boards/173808/cards/5695