Prevent stylesheets from adding twice (or more!)

The problem:

![loadCSS fix by adam mcarthur](http://imgkk.com/i/-lyh.png)

This fix removes the existing stylesheet (by checking the href parameter against the one passed to the `loadCSS()` function) before the new one is added to the DOM.

Adam :)
store each stylesheet in the DOM inside a `current_ss` variable