* Refactored a whole load of crap (as usual)
* Most systems are now AbstractListSystems since they don't really need
tree traversal for nothing
* Fixed some rendering issues related to non-textured objects: Things
with missing textures will now be renderer with an error texture since
textureless entities kind of break rendering

That was the insightful commit message of the day. I should probably ask
someone more experienced about the correct amount of info to provide in
commit messages. Even if I'm working on my own, this should prove useful
sometime.
+ Created a CollisionSystem for collision detection and processing.
+ Implemented a very dirty QuadTree from scratch to make a pretty and
eficient collision detection algorithm.