Add "private" label to non-public items

The Public checkbox is submitted via ajax so this adds a function as
a 'success' callback to toggle the label in the thumbnail-image div.

Fixes #246 

![screen shot 2014-02-19 at 10 01 45 pm](https://f.cloud.github.com/assets/96776/2215543/a176999e-99f4-11e3-9617-c698f27a4758.png)