Use naturalWidth and naturalHeight of image in `presize()` to ensure correct image dimensions

`presize()` was initializing with the wrong dimensions when using an extremely large image (1800x5765).  

So,  `$obj.width()` becomes `$obj.naturalWidth()`, and `$obj.height()` becomes `$obj.naturalHeight()`.

`naturalWidth` is available in modern browsers, but I've  included the jQuery polyfill for `naturalWidth` directly in the Jcrop source... there might be a better way to include that.

Finally, thanks for creating Jcrop, it's quite handy! If you need help writing some unit tests for this so future pull-requests can more easily be validated then let me know.