Added SQS queue policy for SNS subscriptions. (#55)
Added SQS queue policy for SNS subscriptions. (#55)

* Added SQS queue policy for SNS subscriptions.

* Addressed suggestions from @mhahn.

- Moved subscription creation into a function.
- Documented the purpose of the function via docstring.
- Labeled what the portions of split_endpoint are for ARN -> URL
  transformation.

* Cleaned up code using the new from_dict in troposphere.

* Cleanup on the subscriptions switch.

remind101/stacker_blueprints,remind101/stacker_blueprints