[FEATURE] Add "srcset" attribute for images

The "srcset" attribute of the `<img>` tag is part of the [W3C Editor's Draft of HTML5](http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#attr-img-srcset) and is already supported by Chrome 34+ and will be supported by Safari 8. Therefore I think this attribute should be supported by the Flux' image ViewHelpers.