Customize pagination and callback parameters

- [x] Allow to set custom id, title and href on pagination items.
Use data-slidejs-id, data-slidejs-title and data-slidejs-href on each childnodes of the slide

Example here : http://dwarf-labs.com
```html
<div class='slide'>
    <div data-slidejs-id="cartier_winter_tale" data-slidejs-title="Cartier winter tale" data-slidejs-href="cartier_winter_tale">
        <img class='slide-item-img slidesjs-slide-inner' alt='Cartier winter tale' src='pages/films/01_cartier_winter_tale/front.jpg'/>
    </div>
    <div data-slidejs-id="lune" data-slidejs-title="Lune" data-slidejs-href="lune">
        <img class='slide-item-img slidesjs-slide-inner' alt='Lune' src='pages/films/02_lune/front.jpg'/>
    </div>
    <div  data-slidejs-id="maelynn" data-slidejs-title="Maelynn" data-slidejs-href="maelynn">
        <img class='slide-item-img slidesjs-slide-inner' alt='Maelynn' src='pages/films/03_maelynn/front.jpg'/>
    </div>    
</div>
```

result of pagination list :
```html
<ul class="slidesjs-pagination">
    <li id="cartier_winter_tale" class="slidesjs-pagination-item">
        <a href="cartier_winter_tale" data-slidesjs-item="0" class="">
            <span>Cartier winter tale</span>
        </a>
    </li>
    <li id="lune" class="slidesjs-pagination-item">
        <a href="lune" data-slidesjs-item="1" class="active">
            <span>Lune</span>
        </a>
    </li>
    <li id="maelynn" class="slidesjs-pagination-item">
        <a href="maelynn" data-slidesjs-item="2">
            <span>Maelynn</span>
        </a>
    </li>
</ul>
```

- [x] Send current and next slide indexes for each start and complete callback calls.

```javascript
function onSlideChangeStart(current, next){
    ...
}
$('.slide').slidejs({
    callback : {
        start : onSlideChangeStart
    }
});
```