download button added

Add a small downloadbutton next to the close button


new option:
```
downloadIndicator: 'href'
```

downloadIndicator allows to use a orig File as download

example:
```
<a rel="gallery-1" href="big/image2.jpg" class="swipebox" orig="original/image2.jpg">
    <img src="small/image2.jpg" alt="image">
</a>

<script type="text/javascript">
        jQuery(function($) {
            $(".swipebox").swipebox({
                downloadIndicator : 'orig'
            });
        });
</script>
```