NUnit integration

https://github.com/OpenRA/OpenRA/issues/3268
Please change this to `4.0`, otherwise MD4 can't open the solution:

![screenshot from 2014-11-02 16 01 13](https://cloud.githubusercontent.com/assets/4331210/4875680/9d98cca8-62a2-11e4-8b8d-65a97d0325c7.png)