Added srcset support to define pixel-ratio images

This commit adds support for the srcset attribute on images, used to define pixel-ratio images.
The thumbnail is defined as per the standard: http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#attr-img-srcset
Like this: <img src="foo.png" srcset="foo_double.png 2x"... however this has no influence on fancybox
To define the srcset of the large image data-srcset attribute must be set on the anchor tag like this: <a href="bar.png" data-srcset="bar_double.png 2x"...

A full example could be like this:
<a href="bar.png" data-srcset="bar_2.png 2x, bar_3.png 3x, bar_4.png 4x">
    <img src="foo.png" srcset="foo_2.png 2x, foo_3.png 3x, foo_4.png 4x">
</a>