This resolves #77

• Introduces a new client property simianarmy.client.aws.assumeRoleArn
  • which is consumed in the BasicSimianArmyContext
• Takes advantage of existing ability to pass in an AWS credentials provider for AWS client creation
  • which was available but not used anywhere, now all clients are created with a defined provider, the default chain being used as standard
• Introduces a new credentials provider that handles management of the temporary credentials for the assumed role, particularly for renewal when they expire : required as no current AWS SDK credentials provider is suitable
• Alters Conformity Monkey rules that were creating clients to use the appropriate credentials provider
  • ideally they wouldn't have to know about credentials providers but as they are creating clients they have to
  • a future change could address this but it is a more general change, not just to support this feature