Fixed stretched figure images

Some of the figure images are larger than the 150px space allotted them, and while we constrain their width with a CSS max-width, we're also setting their width and height attributes on the img tag to their native width and height. The result is a stretched aspect ratio. You can see this clearly at http://www.html5rocks.com/en/features/storage.

This patch fixes the issue by adding a height: auto to the CSS rule that contains the max-width.