Change data-confirm dialog implementation to callback style.

To create custom dialog for `yii.confirm`, current design is so inconvenient.

JSe's standard `confirm()` function can block event handling until user input. But most of custom modal UI (for example using Bootbox.js http://bootboxjs.com/) can't control script processing. I suggest to change handling `data-confirm` and `data-method` from return-style to callback-style.

I've succeeded to create my dialog box using Bootstrap's Modal component by my callback version.

```html
<div id="my-confirm" class="modal fade">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-body">
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
                <p></p>
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Cancel</button>
                <button type="button" class="btn btn-primary confirm-ok">OK</button>
            </div>
        </div>
    </div>
</div>
```

```javascript
var $confirmModal = jQuery("#my-confirm");

$confirmModal.find(".confirm-ok").on("click", function() {
    var action = $confirmModal.data("ok-action");
    if (action !== undefined) {
        $confirmModal.on("hidden.bs.modal", function() {
            $confirmModal.off("hidden.bs.modal", arguments.callee);
            action();
        });
    }
    $confirmModal.modal("hide");
});

yii.confirm = function(message, action) {
    $confirmModal.find(".modal-body p").text(message);
    $confirmModal.data("ok-action", action);
    $confirmModal.modal("show");
};
```