Rounded Corner Sass Mixin

//defaults all corners to 3px.

=rounded_corners(!tleft = 3px , !tright = 3px , !bright = 3px , !bleft = 3px)
  -moz-border-radius-topleft = !tleft
  -moz-border-radius-topright = !tright
  -moz-border-radius-bottomright = !bright
  -moz-border-radius-bottomleft = !bleft
  -webkit-border-top-left-radius = !tleft
  -webkit-border-top-right-radius = !tright
  -webkit-border-bottom-right-radius = !bright
  -webkit-border-bottom-left-radius = !bleft