Forking: record the parent and show the reference in the UI

It got implemented github-style as of now But it is also supported in the list, too. See picture.

![bildschirmfoto 2013-07-17 um 00 19 15](https://f.cloud.github.com/assets/40496/808544/3ea82466-ee66-11e2-815b-17a44f65e0b5.png)


**Don't forget to also run db:migrate after applying as this contains database changes**