style.css: Standardize fonts

Also, add some common Windows and Mac OS X fonts to the font-family
hierarchies.
functions.php: Remove web fonts

Web fonts are cool, but semi evil*.  Instead we will request several
common, good-looking fonts from Linux, Mac OS X, and Windows and
just hope for the best.  Otherwise we just default to the system's
default serif, sans-serif and monospace.

*try loading a website which uses web fonts on a slow connection!