Mirror video in fullscreen.

[MisterDA]

All that is needed is a parent node to the video, and request fullscreen on that node instead of the video. Tested on latest versions of Chrome and Firefox on Android and Linux. Fix #4.

[jech]

I'm not comfortable with the use of parentNode, I feel it's fragile, it will break as soon as we change the DOM structure. I've done a fair amount of work in the pre-0.1 times to replace all DOM walks with calls to getElementById, as the code becomes more complex, it becomes ever more important to avoid strong coupling between galene.html and galene.js.