Upgraded callback system. Added a 'reload' method to be used in callback...

First of all many thanks for this Krakatoa! I'd like to share some hopefully useful hacks I've done:

- a callback can now be passed as a function reference
- the callback receives a parameter referencing the current instance of the Krakatoa object
- a 'reload' method has been added to reinitialize the sliders from within a callback.

I used all of this as below to reinitialize sliders when browser resolution changes:

```javascript
$('.slider').krakatoa(
    width : '100%'
    , height : 'max'
    , callback : function(slider) {
        $( window ).on('resize', function() {
            slider.reload();
        });
    }
);
```