Add better support for IAM policies for SQS and SNS.

IAM:
  - Added policy constants for common keys and actions.
  - Added Principal helper function for policy Principal list.
  - Added Statement class for policy statements.
  - Add PolicyDocument class for policy documents.

SNS:
  - Added policy constants for SNS specific keys and actions.

SQS:
  - Added policy constants for SQS specific actions.

TODO: Still need to add all the policy stuff for S3.