Adding groupBy.

underscore.js

@@ -250,6 +250,21 @@
       return a < b ? -1 : a > b ? 1 : 0;
     }), 'value');
   };
+  
+  // Groups the object's values by a criterion produced by an iterator
+  _.groupBy = function(obj, iterator) {
+    var result = {};
+    each(obj, function(value) {
+      var key = iterator(value);
+      if (result.hasOwnProperty(key)) {
+        result[key].push(value);
+      }
+      else {
+        result[key] = [value];
+      }
+    });
+    return result;
+  }
 
   // Use a comparator function to figure out at what index an object should
   // be inserted so as to maintain order. Uses binary search.