Picturefill: A <picture> element polyfill variation

Variation of Scott Jehl's Picture Fill, which adds:

<alt>A giant stone face at the Bayon temple in Angkor Thom, Cambodia</alt>
<desc><a href="external/carvingdesc.html">Long description: Giant Stone Face</a></desc>
A giant stone face at the Bayon temple in Angkor Thom, Cambodia Long description: Giant Stone Face