Using srcset attribute with images for more responsive (develop branch)

This make possiple to use new img attribite "srcset". You can use it with thumbs or without.
Example with thumbs:
`<a href="img/large.jpg" srcset="img/large.jpg 1920w, img/medium.jpg 960w, img/little.jpg 480w">
    <img src="img/thumb.jpg">
</a>`
Example without thumbs:
`<img src="img/large.jpg" srcset="img/large.jpg 1920w, img/medium.jpg 960w, img/little.jpg 480w">`