Updated 'highlight' and 'unhighlight' to be less restrictive / Fixed bug with error class not removing properly

I have updated the highlight and unhighlight functions to use the JQuery.closest() method instead of JQuery.parents() as this means it works if the control-group is not the immediate parent.

Also, I found a bug where the highlighting did not get removed properly if you have modified the errorClass option when initialising JQuery.validate.