do not calculate size with hidden elements when resizing

just a small fix when there's a hidden element (like invisible spinner toggled on demand) which causes $resizableElement to be smaller than it can be (and leaves weird space there)

Before:
![image](https://cloud.githubusercontent.com/assets/1067319/2621134/7d572a1e-bc4b-11e3-8b26-deaa7e228cc0.png)

After:
![image](https://cloud.githubusercontent.com/assets/1067319/2621131/64684bbe-bc4b-11e3-92a4-3733ac8a64e7.png)

cc @peterflynn @RaymondLim