Support structural pseudo-classes

[eush77]

Spec: http://www.w3.org/TR/2011/REC-css3-selectors-20110929/#structural-pseudos

• 6.6.5.1. :root pseudo-class
• 6.6.5.2. :nth-child() pseudo-class
• 6.6.5.3. :nth-last-child() pseudo-class
• 6.6.5.4. :nth-of-type() pseudo-class
• 6.6.5.5. :nth-last-of-type() pseudo-class
• 6.6.5.6. :first-child pseudo-class
• 6.6.5.7. :last-child pseudo-class
• 6.6.5.8. :first-of-type pseudo-class
• 6.6.5.9. :last-of-type pseudo-class
• 6.6.5.10. :only-child pseudo-class
• 6.6.5.11. :only-of-type pseudo-class
• 6.6.5.12. :empty pseudo-class