Show next/prev buttons on small screens

On a roll here.

Rather than hide the next/prev buttons on small screens, position them
statically at the bottom of each post and always show the "newer" / "older"
text since there is no hover on touch devices.

This makes for nicer navigation on small devices.

![screenshot 2014-03-10 21 05 32](https://f.cloud.github.com/assets/29612/2381997/2d256f78-a8d3-11e3-9f63-8981742fa8a6.png)