Feature remove credentials

In order to allow usage without giving AWS credentials Ntswf must be given an initialized AWS::SimpleWorkflow object via config[:swf]

For backward compatibility the config[:swf] option is not mandatory yet. The support for aws_access_key_id and secret_access_key options should be removed, when all clients are updated.

Clients that need support for AWS credentials should initialize with Ntswf.create(swf: AWS::SimpleWorkflow.new(access_key_id: 'xxx', secret_access_key: 'xxx')).