Better Timer, dt returns real deltaTime, Add visibility API, resizing&fullScreen fix

- A more precise timer with performance.now()
- dt returns always 0.015, changed it to return the real deltatime since the last frame.
- fullscreenAPI does not need webkit prefix, so I deleted it. (but it does not seem to work either way
- Canvas now always reflects it's real size on the screen
- Visibility API to detect if the Canvas is visible on the screen