rename clients to notification_provider_clients

i think it's causing havoc with my attempts to mock stuff in the
`app.clients` directory because it's also accessible at that path. the
name's super vague and doesn't explain what it is anyway

app/clients/__init__.py

@@ -17,7 +17,7 @@ STATISTICS_DELIVERED = 'delivered'
 STATISTICS_FAILURE = 'failure'
 
 
-class Clients(object):
+class NotificationProviderClients(object):
     sms_clients = {}
     email_clients = {}