Submodulize SFML reference

Using [Git Submodules](http://git-scm.com/book/en/Git-Tools-Submodules) I removed the hard-copy of SFML and use a submodule reference instead. This makes the project much simpler to work with in many ways.

When you clone the repo, you have to run `git submodule init` if you want to have files in lib/SFML, otherwise it is just an empty folder (which is fine if you don't need it for what you're doing)