Add CH hint to @oncletom Pixel ratio implementation 

Hi

I've added the ability to send a CH-DPR header to give the DPR following https://github.com/igrigorik/http-client-hints/blob/master/draft-grigorik-http-client-hints-01.txt. 

I did not remove the DPR from the url because it can still be useful when the server only allows usage of pregenerated static files. Having the DPR in the url also allows you to extract data from access logs to see the % of clients using hidpi clients.

But one thing to keep in mind with the url approach is that some devices have very high DPRs. XPeriaS and Galaxy S4 reach a DPR of 3, not sure if this is a good idea to serve 3x flat files on a 3G connection because of the url. The CH hint is easier to "dishonor" on the server side for example when a 3G connection is detected.

I'm not familiar with Karma, I don't know how to pass the broken "Imager.js handling {width} in data-src should does not use RegExp anymore " test.

I'm basically sending the pull request to see if this is of any interest to you guys. I still think the CH hint is the best way to send the DPR information. @Integralist , I was the guy at your left at the Responsive Images meeting in Paris, sorry for not contributing earlier ;)