Fix GLFW3 callback AccessViolationException

Addresses [https://github.com/andykorth/Pencil.Gaming/issues/19](https://github.com/andykorth/Pencil.Gaming/issues/19) by adding `UnmanagedFunctionPointer` attribute to delegates.

Fix described [here](http://stackoverflow.com/a/18182462/382780).

I have only tested this on Win7-64.