Change visibility of loaded gifs to default

NOT TESTED!

Only apply `visibility:hidden` to loading gifs. This makes unnecessary having to set `visibility:visible` to gifs that are already loaded, which doesn't work on gifs that were hidden by default. 
This uses the `:not()` pseudo-class which is available on both Firefox 3+ and Chrome 14+ as shown in [css-tricks.com](http://css-tricks.com/almanac/selectors/n/not/#browser-support)

Closes https://github.com/octatone/Gif-Delayer/issues/4