Pass $element to offset top/bottom calc funcs

At the moment to make dynamic top offset calculator on multiple elements, one would write:

``` javascript
$('.my-affix').each(function () {
  var $el = $(this);
  $el.affix({
    offset: {
      top: function () { return $el.offset().top; }
    }
  });
})
```

This patch will allow to:

``` javascript
$('.my-affix').affix({
  offset: {
    top: function ($el) { return $el.offset().top; }
  }
});
```