Adds support for @2x background images for retina displays.

hires-graphic-mixin.scss

// higher resolution images for high resolution displays	
@mixin hires-graphic($hdfile, $hdtype, $hdwidth, $hdheight) {
	$filename: $hdfile + '.' + $hdtype;
	$hdfilename: $hdfile + '@2x.' + $hdtype;
	background-image: url('../images/' + $filename);
	@media print, (-o-min-device-pixel-ratio: 5/4), (-webkit-min-device-pixel-ratio: 1.25), (min-resolution: 120dpi) {
		background-image: url('../images/' + $hdfilename) !important;
		-webkit-background-size: $hdwidth $hdheight;
		background-size: $hdwidth $hdheight;
	}
}