Use naturalWidth/Height to avoid a wrong size on retina-screens

Fixed an issue where retinajs used the wrong width/height of the image. For more details about this problem takle a look at [Issue #83](https://github.com/imulus/retinajs/issues/83).

[naturalWidth and naturalHeight](http://stackoverflow.com/a/12556377/207704) are used with a fallback to the original offsetWidth and offsetHeight. Tested with Safari 7.0.3, Firefox 28.0 and Chrome 33.0.1750.117 on OS X 10.9.2.