Added ability to skip items with disabledClass.  Useful when sorting over hidden/non-visible items.

Items with the class specified by the disabledClass option will be skipped when sorting another item over them.

This is useful if you have a nested list where some items can be filtered/hidden but you still need to be able to properly sort over hidden items and change the nesting levels of visible items.  Without this feature sorting over hidden items behaves erratically when changing nesting levels.

Thanks!