add completed/todo merge migration a la @toebu

migrations/20130128_merge_completed_todo_ids.js

@@ -0,0 +1,15 @@
+db.users.find({ completedIds: { $exists: true } }).forEach(function(user) {
+    var newTodoIds = user.todoIds;
+    user.completedIds.forEach(function(value) {
+        if (newTodoIds.indexOf(value) === -1) {
+            newTodoIds.push(value)
+        }
+    });
+    db.users.update(
+        { _id: user._id },
+        {
+            $set: { todoIds: newTodoIds },
+            $unset: { completedIds: 1 }
+        }
+    );
+});