Issue #309, adding symmetric difference as _.symDifference, and making _.difference accept any number of arguments.

underscore.js

@@ -402,10 +402,19 @@
     });
   };
 
-  // Take the difference between one array and another.
+  // Take the difference between one array and a number of other arrays.
   // Only the elements present in just the first array will remain.
-  _.difference = function(array, other) {
-    return _.filter(array, function(value){ return !_.include(other, value); });
+  _.difference = function(array) {
+    var rest = _.flatten(slice.call(arguments, 1));
+    return _.filter(array, function(value){ return !_.include(rest, value); });
+  };
+
+  // Take the symmetric difference between a list of arrays. Only the elements
+  // present in one of the input arrays will remain.
+  _.symDifference = function() {
+    return _.reduce(arguments, function(memo, array) {
+      return _.union(_.difference(memo, array), _.difference(array, memo));
+    });
   };
 
   // Zip together multiple lists into a single array -- elements that share