Load bare images within an HTML document (#3108)

`./servo http://s27.postimg.org/vqbtrolyr/servo.jpg` works now.

This still won't work with local files, since stuff fetched via `file_loader` don't have the MIME type set.

To get this to work with files we'll need to partially implement [The MIME sniffing spec](http://mimesniff.spec.whatwg.org/) and have use it in the resource loaders whenever a MIME type isn't provided.