Call setters with the element as scope.

Conceptually I prefer how bonzo puts the `el` as argument 0 rather than the index like jQuery does. Duplicating the `el` as the scope facilitates writing code that is compatible with both for the most common usage: using the element and not the index.

This allows:

```js
.addClass(function(el) {
     el = this;
})
```
rather than:

```js
.addClass(function(el) {
     el = typeof el == 'number' ? this : el;
})
```