make the <a> tag the parent of the target to add hrefs to everything

This is a fix for the following sprout:
http://www.share-elm.com/sprout/53b117d1e4b07afa6f982635

As first noticed by Do Bi.

I tried to keep this change pixel perfect but there may be a weird case I overlooked. See if you can break this.
This is required because the <a> becomes the width of the entire parent div (i.e., in `flow down [plainText "small" |> link "example.com", image 800 800 "example.com/test.jpg" |> link "exaple.com"]`, 800 pixels of "small" would be linkable. It is no longer contained within the element so it needs to have the dimensions of the element it will be containing.

In addition, using definite pixel counts for width and height has a minor performance boost in rendering.