Subscription Confirmation Support

Now, Zaqar can post messages to the subscriber once the subscriber is registered. It doesn't need the subscriber to confirm the subscription. Then it will lead a problem that users could send junk information to the subscriber no matter the subscriber want or not. So we should let the subscriber confirm that whether he wants the messages or not before post.

Zaqar now support two kinds of subscriber: webhook and mailto. We should add the confirm feature to both them.