Added missing void elements.

When parsing the examples in http://picture.responsiveimages.org/ with jsdom I noticed that the sequences of `<source>` tags didn't end up as siblings in the parsed DOM -- instead the first tag became the parent of the second etc. Turns out `source` (and a couple of others) are missing in the `emptyTags` hash in `lib/Parser.js`.

Source: http://www.w3.org/TR/html5/syntax.html#void-elements