Fix the "Add comment" button position.

It seems the "Add comment" button is now to the right of the text box, instead of below it (like originally in HN). This PR fixes that.

Before / after screenshot:

![hnes-btn](https://cloud.githubusercontent.com/assets/163961/3068640/05ba2ac6-e291-11e3-8b99-589fcf335762.png)