Fix responsive CSS

Updates markup and styles for rows and columns to fit within smaller sizes without stretching to the edge of the screen or causing scrolling overflow at the page-level.

![preview-thumbnail](https://cloud.githubusercontent.com/assets/109822/4605128/7fc6b9ae-51ce-11e4-803c-66839884c72d.png)

Full-page screenshots:  [before][current-360] and [after][new-360]

(I landed on the page on my phone the other day, and thought it was a quick fix. Thanks for your work!)

[current-360]: https://cloud.githubusercontent.com/assets/109822/4605125/7fa51ce0-51ce-11e4-8e2c-255234ae17d3.png
[new-360]: https://cloud.githubusercontent.com/assets/109822/4605151/261d9034-51cf-11e4-90bc-752108bd19a4.png