added support for customizing an item in the dropdown results

I added an additional function <code>itemPostRender</code> to allow developers to customize particular dropdown results. By default, this just returns the HTML that was originally rendered. In the screenshots, I show some examples of how I leveraged this functionality to add a dividing line and loading indicator. 

I additionally added logic into <code>render</code> to check that the <code>items</code> list is not empty. This is a safeguard, to handle the case where a user potentially returns <code>null</code> for all items' HTML. 

Tested browsers:
Safari 6.0.2
Chrome 23.0.1271.101

![Screen Shot 2013-01-02 at 8 49 42 AM](https://f.cloud.github.com/assets/1937659/38735/de92a2b0-54fd-11e2-8ad9-037fe7ea6ff7.png)
![Screen Shot 2013-01-02 at 8 47 39 AM](https://f.cloud.github.com/assets/1937659/38736/df06f818-54fd-11e2-9ff6-6b0e9e682db1.png)