add picture element and related attributes

This PR adds support for the [picture](http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#the-picture-element) element as well as the related [media](http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#the-source-element-when-used-with-the-picture-element:attr-picture-source-media) and [sizes](http://www.whatwg.org/specs/web-apps/current-work/multipage/edits.html#attr-picture-source-sizes) attributes. The picture element recently landed in Chrome 37 (behind a flag) and can be polyfilled in a wide range of browsers using [picturefill](http://scottjehl.github.io/picturefill/).