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)

Wow, I just probably make that change too quick and maybe a bit late at night hehe. Funny, because I’ve done couple of Chrome extension in the past.