Added asdf support

I packaged my code into three ASDF modules. One for SDL, one for OpenGL, and one for the NeHe example (only one sofar). So I can now do a simple (require 'nehe) followed by a (nehe01:run-app) and it runs.

I also found a way to get rid of the hardcoded path reference to the native library I use for doing the Aqua initilisation. I basically ask ASDF for the path CFFI-SDL, and then append the subpath to the right folder. I.e.:

(merge-pathnames #p"sdl/on-cocoa/"
  (asdf:component-pathname (asdf:find-system :sdl)))

I still need to look at the video-info querying. It would also be nice if I could trigger the Makefile from within ASDF as needed. And I'm also thinking of setting up a sourceforge project page for CFFI-SDL, so that anyone interested can get involved.