865 introduce diffs controller

This is the last part of #865. We should create a more beautiful diff view afterwards, but that's "just cosmetics" (issue 922)

![screen shot 2014-09-23 at 14 22 33](https://cloud.githubusercontent.com/assets/339751/4371782/9e3c5364-431c-11e4-95d9-07eeccfd6266.png)
![screen shot 2014-09-23 at 14 23 17](https://cloud.githubusercontent.com/assets/339751/4371785/a0465916-431c-11e4-8f02-2a0550d8b6eb.png)