Added support for itemscope, itemtype, itemprop attributes.

Related issue: #1126
Documentation: http://schema.org/docs/gs.html

**Sample:**
```html
<div itemScope itemType="http://schema.org/Movie">
  <h1 itemProp="name">Avatar</h1>
  <span>Director: <span itemProp="director">James Cameron</span> (born August 16, 1954)</span>
  <span itemProp="genre">Science fiction</span>
  <a href="../movies/avatar-theatrical-trailer.html" itemProp="trailer">Trailer</a>
</div>
```

(I signed the CLA for what it's worth)