Add an escaping class attribute for fancybox.

Add an escaping class attribute `nofancybox` for fancybox.

Now all links with class nofancybox that would normally get auto-enabled, will be excluded from opening in a FancyBox overlay.

```
<a href="url/to/fullimg.jpg" class="nofancybox"><img src="url/to/thumbnail.jpg" /></a>
```