Merged windows_windows.go into common_windows.go.
Removed commented-out checkbox stuff from controls_windows.go since we'll be sending messages directly to the checkbox instead. We still have to worry about radio buttons, so I'm not merging controls_windows.go into common_windows.go just yet.
Added the CL and some notes to the README since it won't land in Go 1.3. I should probably retool this README...