Added "addItem" option

Adds an item to the end of the list, for example "New Entry". This could be used, for example, to pop a dialog when an item is not found in the list of data. 

Example:

      addItem: {id: '', name: "New Member"}

![additem](https://cloud.githubusercontent.com/assets/234/5490329/4a54a5c2-8696-11e4-846c-a5c3c3f5c01d.png)