set the button as the scope of the hold function

Currently there is no access to the button clicked in the `holdFunction` callback while using `stickyClick`. Example:

```
$('#button').stickyClick(function(unstick) {
    // `this` not available
    console.log($(this).attr('id')); // not working
}
```

This PR fixes this issue by setting the `button clicked` as the scope of the `holdFunction`.