Integrated libpng, added a PngTexture class to load png files easily.
DefaultModel modified, so it'll use a face.png file from the working directory if such is present.
face.png added to run/ folder.
Please note that this loading method is just for testing the library, and it will change REALLY soon to use a TextureLoader in render.
As a new library is added, be sure to run make_dependencies again.
Bit of a cleanup in PngTexture