Fix reload of already loaded on event trigger + optional container watch

Trigger the unveil event caused to load the already loaded images, I added a test to prevent the manipualtion of <img> with the same src attribute as the data watched.
I added a argument "container" to allow specific div watch with a fallback on window.
If you merge this modification, please update the minified file.
Trigger the event "unveil" from outside the plugin still load all images.