Settings element - disableActiveLink

For my current Cake 1.3 project,  I require that the active element is
non-clickable but just displayed as a text inside the <li></li>.
I’ve added a new settings element called disableActiveLink. If this is
set to true the <a> tag will not be added.
My apologies for not adding this to the tests.  I feel that someone else
might find this useful so I thought to commit it anyway.