Add a Omnibar icon if a `.colorblind` CSS rule exist on the page.

The only thing I’m really not sure for now is the image for the icon. I think it’s still relevant but there’s place for improvement.

Something more like this would probably make more sense but really not sure of how this would render at a small size.
![colorblind_icon2-192x192__programview1_8178](https://cloud.githubusercontent.com/assets/151262/3593894/a29607a2-0c8f-11e4-8c64-78edffe22e66.jpg)

this isn't what you think it is :smile: 
https://developer.chrome.com/extensions/manifestVersion