Show next/prev buttons on small screens

Rather than hide the next/prev buttons on small screens, position them
statically at the bottom of each post and show the "newer" / "older"
text since there is no hover on touch devices.

This makes for nicer navigation on small devices.
Vertically align next/prev with content
Float #prev to vertically align with #next
Merge pull request #13 from ssorallen/small-screen-next-prev

Show next/prev buttons on small screens