re-arrange directory structure

The directory structure had to be changed to work with a current build of the Dagon engine (tested on Linux). I've also added a script that downloads the actual game content and re-arranges that directory structure too (also keep in mind that Linux is lower-case sensitive). The main reason for this script is that the required libraries for the engine binary aren't available in the right version on every distribution (especially libglew) and compiling it from source is much easier.