Added generic .divider class that can be used with any list-item

#265 

Usage:
```html
<ul id='dropdown1' class='dropdown-content'>
  <li><a href="#!">one</a></li>
  <li><a href="#!">two</a></li>
  <li class="divider"></li>
  <li><a href="#!">three</a></li>
</ul>
```

How it looks like:
![Demo](http://i.imgur.com/Uib8zaV.png?1)

Updated the documentation to show how to use `.divider` too!