Add missing status value for adding members to list
Add missing status value for adding members to list

The docs says the `status` and `status_if_new` can take 5 values, while only 4 are possible in this library. 
http://developer.mailchimp.com/documentation/mailchimp/reference/lists/members/#edit-put_lists_list_id_members_subscriber_hash
charlesthk/python-mailchimp