FEATURE: handler listeners

Fixes ansible/proposals#8

lib/ansible/playbook/handler.py

@@ -20,11 +20,13 @@ from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
 from ansible.errors import AnsibleError
-#from ansible.inventory.host import Host
+from ansible.playbook.attribute import FieldAttribute
 from ansible.playbook.task import Task
 
 class Handler(Task):
 
+    _listen = FieldAttribute(isa='list')
+
     def __init__(self, block=None, role=None, task_include=None):
         self._flagged_hosts = []