Added option 'exposeClass' for the data-options in the <li> elements

New option for adding an extra class to the joyride exposed elements ('div.joyride-expose-wrapper' and 'div.joyride-expose-cover'). This is usefull for styling particular exposed elements.

e.g.: In my first tip, I want it to have its background blue instead of the default style.
usage:
```javascript
<ol>
  <li data-id="my-first-tip" data-options="exposeClass:blue-background">
    First tip...
  </li>
</ol>
```