Added Makefile for MacOS

I added a Makefile to help build the OpenGL example for MacOS Mavericks. I wouldn't expect you to include these changes directly, but having some kind of build instructions would be helpful for people that want to play around with the examples.

Two small things that came up were setting up an OpenGL 3.2 context (not an OpenGL developer, just following hints from Google) and defining nullptr.

This is working for me, though it is linking against homebrew's `glew` and `glfw3`, not the ones included in the repo.