Add high resolution images for retina display

[jes-sherborne]

The new icons look awesome, but they look somewhat less awesome on a device with a high density display (like the retina display on a new iPad). I'm thinking that we should get a double-resolution version of the icon png file and use media queries to selectively alter the CSS when the pixel density is >=2.

[mchesta]

I haven't had a chance to look at the new icons on the iPad(3) yet, but it makes sense that would be an issue. This discussion has come before, if we can't get double-sized versions, I'm pretty sure we have the source vector to do with as we see fit.

We have started integrating CSS conditionals to detect between the retina/non-retina displays for other issues. Currently using a max-width to detect iPad2, and a pixel-ratio check to detect retina displays.