adding resizable class and mixin

lib/mixins.less

@@ -135,6 +135,12 @@
           column-gap: @columnGap;
 }
 
+// Make any element resizable for prototyping
+.resizable(@direction: both) {
+  resize: @direction; // Options are horizontal, vertical, both
+  overflow: auto; // Safari fix
+}
+
 // Add an alphatransparency value to any background or border color (via Elyse Holladay)
 #translucent {
   .background(@color: @white, @alpha: 1) {