Re-Writing Amazon SNS Service Hook #416

Hi all,

After reading [this issue](https://github.com/github/github-services/pull/416), I have taken the time to re-write this service hook so that it is more concise and *works* by attempting to return meaningful errors with respect to configuration with the Amazon service.

* This hook now enables you to define the region that your SNS topic is in. Use the id of the region, ie. "eu-west-1".
* Thinking of those that currently use this hook - if no region is defined, the region will default to us-east-1.
* Use a locked down IAM Resource with an AWS key and secret.
* Better documentation outlining this service hooks usage.
* More test coverage for the new functionality.

I have removed the SQS Queue integration, as I spent quite a while trying different configuration sets from the console and simply could not get this to work. Furthermore, I cannot think of any logical reason as to why you would want a service hook to create an SQS Queue and subscribe it to the SNS topic; especially considering there is a service hook for that now. However, please feel free to implement this feature if you disagree with me.