Add glfwSetDropCallback() support (for GLFW 3.1).

The cgo to Go type conversion in `goDropCB` is unfinished and needs to be improved. Help is highly welcome.

It seems to work on OS X 10.9 (but this doesn't guarantee that the unsafe pointer manipulation is fully correct).

The TODOs should be taken care of and this can be merged when GLFW 3.1 is released.