Add method to erase the todo list.

This should not be done with TodoList.delete(), since it may
remove dependency information.

lib/TodoList.py

@@ -155,6 +155,16 @@ class TodoList(object):
 
         self.dirty = True
 
+    def erase(self):
+        """
+        Erases all todos from the list.
+        Not done with self.delete to prevent dependencies disappearing from the
+        todo items.
+        """
+
+        self._todos = []
+        self.dirty = True
+
     def count(self):
         """ Returns the number of todos on this list. """
         return len(self._todos)

test/TodoListTest.py

@@ -175,6 +175,12 @@ class TodoListTester(unittest.TestCase):
 
         self.assertFalse(self.todolist.is_dirty())
 
+    def todo_erase(self):
+        self.todolist.erase()
+
+        self.assertEquals(self.todolist.count(), 0)
+        self.assertTrue(self.todolist.is_dirty())
+
 class TodoListDependencyTester(unittest.TestCase):
     def setUp(self):
         self.todolist = TodoList.TodoList([])