Added fixed width class from FontAwesome.

less/_fixed-width.less

@@ -0,0 +1,10 @@
+// fixed width icons
+// --------------------------
+
+// lifted from FontAwesome
+// https://github.com/FortAwesome/Font-Awesome/blob/6549e2920b0a27213b895200becc485ab01e9974/less/fixed-width.less
+.@{wi-css-prefix}-fw {
+
+  text-align: center;
+  width: (18em / 14);
+}

scss/_fixed-width.scss

@@ -0,0 +1,10 @@
+// fixed width icons
+// --------------------------
+
+// lifted from FontAwesome
+// https://github.com/FortAwesome/Font-Awesome/blob/6549e2920b0a27213b895200becc485ab01e9974/less/fixed-width.less
+.#{$wi-css-prefix}-fw {
+
+  text-align: center;
+  width: (18em / 14);
+}