Feature/picturefill

This is not actually ready to be merged into master, but I wanted to give you access to it now to review what's going on.

Basically, this work implements picturefill ( https://github.com/scottjehl/picturefill ) as a solution to serving proper images. There's a new [picturefill] shortcode, which actually gets used when utilizing the "Add Media" tool in the WYSIWYG. I believe the gallery is also utilizing picturefill as well.

The still-to-be-resolved issues:
1. Something in Argo Audio Player is messing things up. You'll need to turn that plugin off for things to work. 
2. Still need to fix the slideshow plugin to support the newfangled ids=1,2,3,4 argument for gallery shortcodes.

Also a question:
right now the picturefill shortcode stuff, etc is mostly in functions.php — this seemed like sort of a core thing to have in the theme. Is that okay or would you prefer it be broken out into an inc/ file?