Add Subscription Confirmation

[kevineaton]

As new SNS listeners come on line, when adding to a topic they will be sent an SNS message to confirm their subscription. Although probably beyond the initial scope of this library, it would be helpful to automatically approve the subscription.

Currently, if a raw message is passed directly in, and it is a Subscription Confirmation, the library will call the onError handler since the message body is not JSON.

[kevineaton]

Fixed in #19