Use the allowfullscreen attribute in addition to webkitallowfullscreen

Using only the prefixed attribute will break in Chromium:
https://code.google.com/p/chromium/issues/detail?id=390008
https://code.google.com/p/chromium/issues/detail?id=391695
Merge pull request #1 from foolip/allowfullscreen

Use the allowfullscreen attribute in addition to webkitallowfullscreen