SASS sprite mixin

//**
  Example use:
  a
    +sprite-bg("icons-32.png")
  a.twitter
    +sprite-column(1)
  a.facebook
    +sprite-column(2)
  ...

!sprite_default_size  ||= 32px
!sprite_default_margin ||= 0px

// Sets rules common for all sprites
=sprite-bg(!img, !width = !sprite_default_size, !height = !width)
  :background= url(!img) "no-repeat"
  :width= !width
  :height= !height
  :overflow= "hidden"

// Allows horizontal sprite positioning
=sprite-column(!col, !width = !sprite_default_size, !margin = !sprite_default_margin)
  !x = ((!col - 1) * -!width) - ((!col - 1) * !margin)
  :background-position= !x 0

// Allows vertical sprite positioning
=sprite-row(!row, !height = !sprite_default_size, !margin = !sprite_default_margin)
  !y = ((!row - 1) * -!height) - ((!row - 1) * !margin)
  :background-position= 0 !y