WIP: Fix callbacks after ~fn removal

I'm having trouble building the examples manually with rustc (rustpkg is being a pain). I get:

~~~
$ cd src/glfw
$ rustc lib.rs
$ cd ../examples/window
$ rustc -L ../../src/glfw main.rs
main.rs:1:0: 1:0 error: can't find crate for `std`
~~~

This should ultimately fix #48.