Add credits.html for mac for clean about dialog

Added a credits.html file so that grunt-node-webkit-builder doesn't generate the default one.
Since the credits.html is empty, it makes a clean about dialog: 
![screen shot 2014-06-23 at 2 11 03 pm](https://cloud.githubusercontent.com/assets/4448627/3362486/42066fda-fb0a-11e3-9be3-2cddb407bc17.png)