Make the video element responsive along with the img

[chris-pearce]

backpack.css/src/image-and-multimedia.css

Lines 1 to 12 in 318df58

	/**
	* Make all images responsive.
	*
	* 1. Override the height to 'auto', otherwise images will be stretched when
	* setting a width and height attribute on the `<img>` element.
	* 2. Set a maximum relative to the parent.
	*/
	img {
	height: auto; /* 1 */
	max-width: 100%; /* 2 */
	}

[chris-pearce]

Addressed in #40.