[orbit] Adding ability to select first image shown in orbit via active_slide_class

For example: 
```html
<ul data-orbit>
  <li>
     <img src="..."/>
  </li>
  <li class="active">
    <img src="..."/>
  </li>
</ul>
```
Will show the second image after orbit init.

Any feedback is appreciated. There is still an animation from the first image to the image that is selected. I'm not sure of how to get rid of this unless since active_slide_class can't be exposed in the scss. Any ideas?