Serve a high-DPI version of images

[Wonskcalb]

As we discussed on issue #59, we've got to be careful with "Retina" devices, which they all pretty much are on the day of writing this. Issue #59 was more about shrinking down images resolution than DPI, that's why I'm opening this here. Resolution and DPI are correlated, but not indentical. And by shrinking images' resolution down, we might end up someday with poorly detailed images, as screen's pixel density keep increasing.

In that matter, I suggest we serve two versions of images (1x and 2x) more informations can be found on this website. But for now, we should generate media-queries to grab the correct 1x or 2x image, as for now, images-set are only suppported in webkit based browser