add custom support for genesis themes

A couple of months ago, I switched to use [Genesis](http://my.studiopress.com/themes/genesis/) on my personal site.  One of the things I really like about it is that it is incredibly extensible, with hooks to modify the attributes of various elements throughout the theme.  This actually allows for a much cleaner approach to adding microformats, since you don't need to use wrapper divs.

I was hesitant to send this as a pull request at all, since I'm not sure if it really makes sense to add such custom support for one particular theme framework.  But I figured I'd at least send it your way and see what you thought.  For what it's worth, this is at least well behaved in the sense that it only loads genesis.php if genesis is active.