Adds a 192x192 icon, uses new "theme color"

This is inspired by [this article](http://updates.html5rocks.com/2014/11/Support-for-theme-color-in-Chrome-39-for-Android) in HTML5Rocks, which suggests a `192x192` png for the main `rel="icon"` tag, and advertises a new meta tag for a "theme color":

```
<meta name="theme-color" content="#1188ff">
```

I added this, and in the task switcher it looks great, and blends in perfectly with our high res favicon:

![screenshot_2014-11-23-13-24-45](https://cloud.githubusercontent.com/assets/4592/5158704/40248ab6-7315-11e4-8f02-e36d06b1c9f4.png)

As the Chrome header bar color, it looks less great, especially with the green of the `https://` marker:

![screenshot_2014-11-23-13-25-02](https://cloud.githubusercontent.com/assets/4592/5158706/45afc234-7315-11e4-9d1c-94a4e327e5da.png)

Ideas welcome.