Added AWS SNS Notification Gateway

Hello all,

This is my first draft of a notification gateway that delivers to [Amazon's SNS service](https://aws.amazon.com/sns/).  Not being Australian, MessageNet doesn't really make sense for me, so I decided to give it a whirl.

SNS allows users to send bulk SMS messages and emails at a relatively decent price (I'm not sure why you'd want to use SNS's email capabilities over Flapjack's SMTP gateway, but I digress). 

This PR adds an `sns` media type where the user can define their own individual Topic ARN, which allows user to separate topics among teams and/or individuals. It's expected that all users will be able to share an Access/Secret Key pair.

Some things to discuss:

* Would it be a better idea to create an 'abstract' SMS gateway, which has individual drivers? For example, a user could configure the SMS gateway to use either the Messagenet or SNS driver and provide credentials for the selected driver.  This might get hairy pretty quickly as they're not entirely interoperable.
* Would it be at all beneficial to allow users to have individual Access/Secret key pairs?  I think this would probably be accomplished along the same way that PagerDuty credentials are handled, but it'd  definitely require more effort.  Perhaps we could come up with something akin to a [DSN](https://en.wikipedia.org/wiki/Data_source_name) where a user could specify `sns://key:secret@region/topic_arn` as their SNS media.  However, that would also add complexity.
* Finally, I've implemented [version 2 signatures](http://docs.aws.amazon.com/general/latest/gr/signature-version-2.html) directly within the gateway.  I didn't feel it would be appropriate to add a dependency to the [official SDK](https://github.com/aws/aws-sdk-ruby) since it would both add bulk and works outside the EM cycle.

Please discuss and let me know what I can do to make this ready to merge!