Add a close function

I know this project hasn't been touched in several years but I figured this would be a helpful function, I know I use it frequently and it appears that an attempt was previously made to include it.

I am uncertain as to why the win32gui.CloseWindow function didn't work but I was able to acheive the desired result using win32gui.SendMessage.

I recognize that this may not be the best place to include the change, but I figure its quite minor and most people (at least those on Windows that this will affect) will be using this package as opposed to the one on launchpad.