Added srcset support

[milansimek]

As discussed in issue #35

[pau1phi11ips]

This makes more sense if added this way

var source = this.getAttribute(attrib);
source = source || this.getAttribute("data-src");
if (source) {
	if (this.tagName === 'IMG') {
		this.setAttribute("src", source);
	} else if (this.tagName === 'SOURCE') { // add srcset support
		this.setAttribute("srcset", source);
	} else {
		this.style.backgroundImage = 'url(' + source + ')';
	}
	if (typeof callback === "function") {
		callback.call(this);
	}
}

[milansimek]

No it doesn't. My pull request is for srcset attributes, not for source tags inside a picture tag.