Add support for an AWS SNS Publisher

[mclaughlinseanp]

Sending notifications via the email publisher while hosted on AWS EC2 instances is problematic due to the need for email server admins to configure relays through the remote VM(s) - this is so much of a security concern for the a lot of admins that they often simply refuse to allow it.

Providing the ability to do notify through an AWS SNS Topic (in the same way as AWS CloudWatch alarms do) through a new type of publisher would be a great addition to the tool and would solve this security concern nicely.

I am actually intending to work on this soon and will issue a pull request for you to review if you would like at some point in the next week or so.

[mbok]

Looking forward for your contribution! It'll be a great extension.