SDL window

Created by: dakk

I was watching your project (great work guys), and I see you use X11 (on Linux) and Cocoa (on OSX) for the window creation, and for input events; it could be a better solution to use an SDL backend for that, which is compatible with all major OS ("SDL officially supports Windows, Mac OS X, Linux, iOS, and Android" http://libsdl.org). There are already few sdl-wrappers on Opam.