Add Opera-prefixed radial gradient

Tested and works in Opera 11.61

less/mixins.less

@@ -509,8 +509,8 @@
     background-image: -webkit-radial-gradient(circle, @innerColor, @outerColor);
     background-image: -moz-radial-gradient(circle, @innerColor, @outerColor);
     background-image: -ms-radial-gradient(circle, @innerColor, @outerColor);
+    background-image: -o-radial-gradient(circle, @innerColor, @outerColor);
     background-repeat: no-repeat;
-    // Opera cannot do radial gradients yet
   }
   .striped(@color, @angle: -45deg) {
     background-color: @color;