Fixed WindowMode

- Made FullScreen wrap a borrowed Monitor.
- Removed from_ptr
- Changed ‘get_window_mode’ to take a closure.

Code example to get full screen:
```
          let glfw = glfw::init(glfw::FAIL_ON_ERRORS).unwrap();
          let (window, events) = glfw.get_primary_monitor(|m| {
              glfw.create_window(width, height, title, 
                  glfw::FullScreen(m.unwrap()))
                      .expect("Failed to create GLFW window.")
          });
```