Merge existing SNS topic policy with CloudTrail policy.

This prevents overwriting the default topic policy and makes the CLI behave
the same as the AWS Console.
Prevent printing None for prefix
Pull SNS topic merging into its own method; fix tests.
Reformat example policies
Merge branch 'cloudtrail-policy' into develop

When creating new SNS topics, merge the CloudTrail policy statements
with any existing policy statements.