make the height of avatar unconstrained in post-single page

In the post-single page, the `a` with `post-author-avatar` class element has `width:80px;height:80px;` css rules, but just with `width:100%` of inner image.If user's avatar isn't a square, UI breaks. See:

![screen shot 2014-12-08 at 13 28 58](https://cloud.githubusercontent.com/assets/2620575/5335306/c9b1a174-7ee1-11e4-9e4b-1b8b342b921c.png)

There're 2 solutions: constrain the avatar size width equal height or just constrain the width to 80 pixel. I think the second solution is the best.