Smarter picturefill shortcode

This is a revised version of the picturefill implementation that should work better with the Navis Media Credit plugin, which previously was causing problems with hard-coded <div> element widths. This new picturefill now runs after the media credit plugin, with the picturefill shortcode insertion code now looking for an existing `[caption]` in the $html that's passed to it. If it finds one, it extracts that information and wraps it around the picturefill shortcode that is inserted into the post. If there is no `[caption]` then nothing happens.

The Navis Media Credit (NMC) plugin's shortcode runs with priority 19. The picturefill shortcode insertion must run after this, so it has been changed to priority 20.