Added missing Action import

Added missing import Manager.get_action(id) fails when Action is not
imported

Before import:
```
do_mgr = digitalocean.Manager(token='snowflake')
action_id = 111111 # replace with valid action id.
action = do_mgr.get_action(action_id)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "build\bdist.win32\egg\digitalocean\Manager.py", line 185, in get_action
    return Action.get_object(api_token=self.token, action_id=action_id)
NameError: global name 'Action' is not defined
```