Box2d Update

With this first update, I wanted to iron out a lot of the bugs present in the first version.

To accomplish this, I converted the physics system to run through Box2D, an open source 2d physics simulator. There were quite a few headaches involving the port but I finally got everything ironed out and working well.

The other big changes were in regards to performance. I did some minor optimization to the shader code, but I also implemented a performance mode, which can be enabled through the tilde(`,~) key. This will completely disable the light shader, so I am still looking into compromises that isn't this nuclear option, but if the framerate is too low for you this should help quite a lot.