Add support for picture tag

Background info

• Core issue: #1170478: Responsive Images
• Proposal: http://www.w3.org/community/respimg/wiki/Picture_Element_Proposal
• Discussion: http://www.w3.org/community/respimg/
• Polyfill using div/span: https://github.com/scottjehl/picturefill
• Polyfill using picture https://github.com/Wilto/picturefill-proposal

Todo

• Add an alternative method using picturefill
• Add an alternative method using the <picture> tag

Demo

Drupal 7