Fun with textures

Added SOIL to load images into OpenGL textures, as SFML's own code for
that isn't working quite right.  (Plus, SFML does not give you access to
the texture ID, which is necessary for doing neater stuff like
multitexturing.)
Test rendering with a bound texture works, although it is sloooow (no
doubt due to binding/unbinding the texture on each frame).
Updated the README file to better describe the project.
Dumped the Image class since SOIL takes care of that stuff (whereas with
libpng I would've had to do all that myself).
Updated .gitignore to ignore Code::Blocks temporary files.