Add Spritesmith mixins

[mikestreety]

As we are starting to use spritesmith, would be good if we can fold in the spritesmith mixins - so we can have them with a custom name.

To stop them being generated, put this in gulp file:

twolfson/grunt-spritesmith#63 (comment)

The mixins need to replicate the following:

@mixin sprite-width($sprite) {
  width: nth($sprite, 5);
}

@mixin sprite-height($sprite) {
  height: nth($sprite, 6);
}

@mixin sprite-position($sprite) {
  $sprite-offset-x: nth($sprite, 3);
  $sprite-offset-y: nth($sprite, 4);
  background-position: $sprite-offset-x  $sprite-offset-y;
}

@mixin sprite-image($sprite) {
  $sprite-image: nth($sprite, 9);
  background-image: url(#{$sprite-image});
}

@mixin sprite($sprite) {
  @include sprite-image($sprite);
  @include sprite-position($sprite);
  @include sprite-width($sprite);
  @include sprite-height($sprite);
}```