[WIP] Use messaging instead of trait objects for event handling

Instead of traits this uses ports and channels. Whenever `glfw::poll_events();` is called, the events are sent to window-specific ports, along with the time stamp of when they were triggered. The events can be accessed using either the `Window::events` or `Window::flush_events` methods. For example:

~~~rust
let window = glfw::Window::create(300, 300, "Hello this is window", glfw::Windowed)
    .expect("Failed to create GLFW window.");

window.set_key_polling(true);    // polling must be registered for the specific `WindowEvent` type
window.make_context_current();

while !window.should_close() {
    glfw::poll_events();
    // `flush_events` returns an iterator that empties the messages received since
    // `events` or `flush_events` was last called.
    for (_, event) in window.flush_events() {
        match event {
            glfw::KeyEvent(glfw::KeyEscape, _, glfw::Press, _) => {
                window.set_should_close(true)
            }
            _ => {}
        }
    }
}
~~~