add sass helpers

style/helpers.scss

+// Exponent
+// From: https://github.com/Team-Sass/Sassy-math/blob/master/sass/math.scss#L36
+
+@function exponent($base, $exponent) {
+  // reset value
+  $value: $base;
+  // positive intergers get multiplied
+  @if $exponent > 1 {
+    @for $i from 2 through $exponent {
+      $value: $value * $base; } }
+  // negitive intergers get divided. A number divided by itself is 1
+  @if $exponent < 1 {
+    @for $i from 0 through -$exponent {
+      $value: $value / $base; } }
+  // return the last value written
+  @return $value;
+}
+
+@function pow($base, $exponent) {
+  @return exponent($base, $exponent);
+}