Responsive mixins

common.scss

@mixin respond-to($media) {
  @if $media == iphone {
    @media only screen and (max-width: 640px) { @content }
  }
  @else if $media == ipad {
    @media only screen and (max-width: 980px) { @content }
  }
  @else if $media == ipad-landscape {
    @media only screen and (max-width: 1024px) { @content }
  }
}