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.
yep. added!

On Fri, Sep 19, 2014 at 9:45 AM, Jeffrey Posnick <notifications@github.com>
wrote:

> In google-youtube.html:
>
> > @@ -224,6 +240,41 @@
> >        thumbnail: '',
> >
> >        /**
> > +       * If `fluid` is set, then the player will set its width to 100% to fill
> > +       * the parent container, while adding `padding-top` to preserve the
> > +       * apect ratio provided by `width` and `height`. If `width` and `height`
> > +       * have not been set, the player will fall back to a 16:9 aspect ratio.
> > +       * This is useful for responsive designs where you don't want to
> > +       * introduce letterboxing on your video.
> > +       *
> > +       * @attribute fluid
> > +       * @type boolean
> > +       */
>
> Could you add @default false? (Since it shows up in the
> core-component-page now.)
>
> —
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> <https://github.com/GoogleWebComponents/google-youtube/pull/12/files#r17795325>
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>