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.
Ah, sorry, you're right. I mistakenly thought you were setting a native property of the Polymer element, when it's just setting the published attributes.