Start of picturefill refactor

About two months ago I created a fork of picturefill that implements the latest spec proposed by Tab Atkins:

http://picture.responsiveimages.org/

Opening up this PR now to kickstart the refactor (https://github.com/scottjehl/picturefill/issues/125)! There have been updates to the spec since I last worked on this, so it is very likely that there will be numerous updates needed. Looking forward to the comments and the impending process on getting an official picture polyfill created :). Exciting times ahead!