data-picture: Converted to fully event-driven

1. Ported Picturefull code into `plugin.picturefill` function.  This
   allows the plugin to only enhance the element(s) that trigger the
   `picturefill.wb-data-picture` event.
2. Removed picturefill and matchMedia dependencies (Modernizr.mq is
   now used to match media queries).

@masterbee these are the changes we were talking about earlier today:

* saves the DOM from thrashing, and
* allows any element to use `picturefill.wb-data-picture` event (no init required).