Add glfwPostEmptyEvent() support (for GLFW 3.1).

GLFW 3 has just recently merged a new function `glfwPostEmptyEvent()` into master (what will eventually be released as GLFW 3.1), see https://github.com/glfw/glfw/issues/218#issuecomment-37137200.

This PR adds support for it. It should be merged when GLFW 3.1 is released, because it's incompatible with current GLFW 3.0.4.

I tested that it works [here](https://gist.github.com/shurcooL/5816852/revisions).