Added function shuffle, with test case.

underscore.js

@@ -239,6 +239,21 @@
     return result.value;
   };
 
+  // Shuffle an array.
+  _.shuffle = function(obj) {
+    var shuffled = [], rand;
+    each(obj, function(value, index, list) {
+      if (index == 0) {
+        shuffled[0] = value;
+      } else {
+        rand = Math.floor(Math.random() * (index + 1));
+        shuffled[index] = shuffled[rand];
+        shuffled[rand] = value;
+      }
+    });
+    return shuffled;
+  };
+
   // Sort the object's values by a criterion produced by an iterator.
   _.sortBy = function(obj, iterator, context) {
     return _.pluck(_.map(obj, function(value, index, list) {