Add fluid resizing

I took this from fitvids and fluidvid. If there is a `fluid` attribute set on the `google-youtube` element it will absolutely position and stretch the player to fill the parent container, while giving the parent container enough top padding to force the correct aspect ratio, even as the screen resizes.
Falling back to a 16:9 does sound reasonable to me. It's going to give you what you'd expect for the very large majority of videos from the past ~4 years, and for older videos that fall prey to this edge case, black bars in the playback area isn't a showstopper.