new function to get a pixbuf from a cairo surface

This patch allows calling gdk_pixbuf_get_from_surface() from gtk2hs. It's a gtk3 specific function, I'm putting it at 3.0.0+ but I'm not sure in which GTK version it was added.

I wrote this code half-guessing looking how you wrap similar functions. Maybe propageGError is needed, but I could not make it work.

I have a sample program and it works with it:
https://dl.dropboxusercontent.com/u/22600720/Main.hs

I think it's a really nice function...