Changes notify to use ctypes instead of win32gui so we could use unicode.

In addition to the above, you can also create one instance and call notify as many times as you like with different messages on that instance.

I don't like the fact that we need to sleep inside the `__init__` method, but I left it there for backward compatibility reasons. I could also add a close method which explicitly closes the balloon whenever you wish, but the (plyer notification) api doesn't seem to have an option for this.

Finally, it doesn't show any icon on the systray, just in the balloon, but that could be changed as well to show an icon in the systray as well.