remove 3d translate upon close

Hi, great work with this plugin. I was looking for a decent hardware accelerated option just like this! 

I discovered that if you keep a 3d transform on the main #sb-site div, scrolling performance will be poor in Chrome. (More noticeable on a retina macbook pro) The FPS was dropping well below 30 with this transform. I think it also becomes a lot worse if there are numerous pictures on the page. 

I added a small change to the close function and it just sets the transform to 'none' after closing the sidebar. This restores scrolling performance.