Use Collection#where for TodoList#remaining

Using `Collection#where` for both speed and symmetry.

examples/todos/todos.js

@@ -48,7 +48,7 @@ $(function(){
 
     // Filter down the list to only todo items that are still not finished.
     remaining: function() {
-      return this.without.apply(this, this.done());
+      return this.where({done: false});
     },
 
     // We keep the Todos in sequential order, despite being saved by unordered