Add support for srcset on img and source tag

```
https://www.webkit.org/blog/2910/improved-support-for-high-resolution-displays-w
ith-the-srcset-image-attribute/

bhttp://mobile.smashingmagazine.com/2013/08/21/webkit-implements-srcset-and-why-
its-a-good-thing/
```

Original issue reported on code.google.com by `philippe...@gmail.com` on 8 Sep 2013 at 8:28
