Squashed commit of the following:

commit 81324491f4913d6797c3c9e4df83a1aefd7d9116
Author: Daniel <dano.husar@gmail.com>
Date:   Sat May 10 17:37:29 2014 +0100

    use opacity mixin

commit 694b0a3fcd196e8cba9e4c23850f903c0804cb44
Author: Daniel Husar <dano.husar@gmail.com>
Date:   Fri May 9 13:47:35 2014 +0100

    Update button-groups.less

less/button-groups.less

@@ -232,5 +232,5 @@
 [data-toggle="buttons"] > .btn > input[type="checkbox"] {
   position: absolute;
   z-index: -1;
-  opacity: 0;
+  .opacity(0);
 }