merging #532 -- allow string as iterator on sortBy

underscore.js

@@ -262,7 +262,8 @@
   };
 
   // Sort the object's values by a criterion produced by an iterator.
-  _.sortBy = function(obj, iterator, context) {
+  _.sortBy = function(obj, val, context) {
+    var iterator = _.isFunction(val) ? val : function(obj) { return obj[val]; };
     return _.pluck(_.map(obj, function(value, index, list) {
       return {
         value : value,