Support img srcset & sizes

[acrylian]

Picturefill not only supports <picture> but also <img srcset="">. They seem to do the same but apparently that is not exactly true. <picture> sets fixes images while <img srcset> is more relative and leaves it to the browser what to display when. So in many cases to use this is more clever than <picture> actually.

Recommended reading with lots of details:

• http://www.smashingmagazine.com/2014/05/14/responsive-images-done-right-guide-picture-srcset/
• http://ericportis.com/posts/2014/srcset-sizes/ (I will implement the srcset+sizes way)
• basic standard/high density img srcset support
• basic breakpoint based img srcset & sizes support