Retina sprites compass mixin

_compass_retina_sprites.sass

=retina-sprites($spritesheet)
  @each $sprite in sprite_names($spritesheet)
    &.#{$sprite}
      $positions: ''
      @each $j in sprite-position($spritesheet, $sprite)
        $positions: append($positions, $j / 2, space)
      height: image-height(sprite-file($spritesheet, $sprite)) / 2
      width: image-width(sprite-file($spritesheet, $sprite)) / 2
      background-position: $positions

  background-image: $spritesheet
  background-size: image-width($spritesheet)/2 image-height($spritesheet)/2