>Hi everybody,
>
>I am still not clear how directives work and how to make them optional.

The link function passes a third parameter which lets you get at the attributes:

// Instead of this:
            link: function (scope, element) {
// Do this:
            link: function (scope, element, attrib) {

Now you have access to the attributes, so to grab the value:

var attribValue = attrib.smFocus

(take a look at attrib in the debugger and you'll see the values it holds).