Update jquery.revolver.js

Reduce animations number from 2 to 1 for 'Fade' transition type.

Preload image functionality:
new option - preload: 1/0,
new syntax:
```html
<div id="revolver">
<img class="slide" src="first.image.jpg">
<img class="slide hidden" src="0.gif" data-src="original.image.jpg">
</div>
```
(display:none doesn't disable image loading, because that i'm using this data-src tag)