Whitelist video tag and add "poster" as an acceptable attribute of <video>

Whitelist video tag and add "poster" as an acceptable attribute of <video>
changes wrt: https://code.google.com/p/html5lib/source/detail?spec=svn6a52f08fb7eb81870095b763a50406fc8216054e&r=6a52f08fb7eb81870095b763a50406fc8216054e