Refactoring

When I did #43 and dig into webpage's code, I felt pain.
Let's make Terasology splash site as beatiful as Terasology itself!
You can view demo of these changes [here](http://neoascetic.me/terasology.org/), so please report on any bugs you can find.

I've optimized all pictures (to speed up their loading) using ImageOptim and because of this the PR is so fat in terms of bytes (+3Mb). If you do not want this, I can remove these changes and make a force-push to the branch again to prevent repo from bloating up).