Add SNS permissions for Scout2

[edeca]

Fixes #7

[l01cd3v]

Thanks ! merged.

FYI, In a somewhat near future, I will probably do the following:

• Modify list* into an explicit list of all APIs (just 2 for now)
• Create a new policy (maybe "Scout2-All") that includes ListSubscriptions and remove it from the default policy in case people are not comfortable with exposing emails addresses/phone numbers/... to their auditors. This would happen after modification of Scout2 to handle the not authorized exception.