Adding a "Stockholm-Skybox"

We have integrated a skybox picturing the Stockholm skyline that was used by the previous MVK-group for this project. The colors of this skybox did not match our scene perfectly, so we had to adjusted them slightly. We still have some small fixes to do on this skybox, but this is the preliminary result:

1. Before - The original Stockholm-skybox

2. After tweaking the colors

/ Tom & Mona