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).