If the last item in the object is a function, it will be returned. Otherwise, an "empty" function will be returned.
Useful for ensuring that a callback can always be invoked, without checking if the argument is a function
over and over.
@param {Object} o The object to get the callback for.
@return {Function} If the last item in the object is a function, it will be returned. Otherwise, an "empty" function will be returned.
function getCallback (o) {
                    var last = libs.object.last(o);
                    return last instanceof Function ? last : NULL_FUNCTION;
                }