Favicon support

Added support for favicons and updated docs accordingly.

There are two acceptable mime types for .ico ( http://en.wikipedia.org/wiki/ICO_(file_format)#MIME_type ):
image/vnd.microsoft.icon - official
image/x-icon

I used the most common way of including a favicon ( http://en.wikipedia.org/wiki/Favicon#How_to_use ):
```
<link href="https://cdn.your-site.com/img/favicon.ico" rel="shortcut icon" />
```

This is obviously a bit hairy as a favicon is an image, so maybe in future revisions there could be some specificity about what should be generated.