Issue 1169 - Implement Picture-element and srcset support into Photon

# Adding srcset elements to Photon images

## issue 1169
See: https://github.com/Automattic/jetpack/issues/1169

## Problem
Retina images are being implemented by using devicepx-jetpack.js – in the past it was the best way to determine the user zoom level on every browser. If the user zoom in\zoom out or has a retina, Jetpack will immediately go to Photon and fetch the correct version of the image for the user’s zoom. The downsize of using this script is a function that runs every second – lead to performance issues. Users complained about it and you can find online several posts of developers about it. Google devicepx-jetpack.js for a lot of examples.

## General fix details
Original WordPress "devicepx" hosted file removed and new jetpack-retina-support added. The new file include srcset addition to supported browsers, Fallback is using the original devicepx methods.

## How to test
* Go with Chrome browser (supports srcset) to a page that use images (in the post itself or in the widgets) see that srcest attributes for zoom 1x,2x,3x,4x are being implemented by JavaScript for Photon, Wordpress.com & Gravatar.
* Go with Firefox browser (not supporting srcset), See that it is loading the appropriate image according to zoom level.

## Warning
We are losing functionality here. srcset does not reload the image dynamicly if the user is zooming in or out. 
ZoomImages is being called every second to detect if the user is changing the zooming and change the src of the image according to the zoom.
srcset does not support reloading of images when user is zooming in or out. I though about dropping the zoomImages calls in every second, since it seems not very important feature - especially when you see the amount of complaints over the internet on this. When I raised this issue, @georgestephanis  said (and I agree with him) to avoid feature creeping. Maybe after we implement the srcset support, we can open another issue about dropping the interval that calls zoomImages every second. it seems a very high price for a functionality that serves correct pictures immediatly to people that zoom in\out.