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)

That part show the icon: `chrome.pageAction.show(tabId);`

The only place when you can add a pageAction is from a background page (or script only in our case). That why there is a `chrome.tabs.sendRequest` to send a request to the current tab content to have access to it’s content. In our case we need to access the DOM with our `inject.js` file to see if the `.colorblind` CSS rules is defined in any StyleSheet. So if the `response` of the request to the tabs is true (which is return from my `hasStyle(selector)` function) that means there’s actually a `.colorblind` CSS rules somewhere for the current page.

For reference, a `pageAction` is an icon added into the Omnibox (at the right), and a browserAction is a icon added in the browser at the right of the Omnibox outside of it, like most extension does.