Fix font sizing of monospace fonts

In many browsers, monospaced fonts are displayed at 13px instead of 16px. But that behaviour isn't consistent, so you can't easily fix it.

Zen currently applies a 1.1em font size to monospaced elements, but that's not a very good fix.

I just discovered http://meyerweb.com/eric/thoughts/2010/02/12/fixed-monospace-sizing/

All we need to do is add "sans-serif" to the end of our monospaced font stack.