Fix simple.jl so that it runs on Julia 0.3 on OS X 10.9.3.

- Added two C_NULL arguments to GLFW.CreateWindow; there's only one method defined and it expects monitor::Ptr{None} and share::Ptr{None} arguments
- Check whether the return value of GLFW.WindowShouldClose is zero by == comparison rather than the boolean \!GLFW.WindowShouldClose(window), which throws an error because WindowShouldClose returns an int and ! is defined for Bools and BitArrays.

I'm not sure whether the example *should* work and there's something odd going on with my setup or whether there's an actual issue, but the two small edits in this pull request are required to get the example to run on my computer.