Unified handling of REST service inputs.

Renamed xml and root to payload.  This makes the service
descriptions also able to match the documentation and user
inputs directly.

Fixes #71
Updated unit and integration tests to use payload trait for rest services.

References #71
Added missing options to s3 documentation.
Added recordMarkerFailedEventAttributes to AWS.SimpleWorkflow.Client operations.
Added missing input members to AWS.EC2.Client api.
Added cancelUpdateStack operation to AWS.CloudFormation.Client.
Updated AWS.Redshift.Client (and api documentation).
Removed some required traits from the OpsWorks api configuration.
Regenerated client API documentation in doc-src/.

* Removed required traits from response descriptions
* Corrected nesting in REST-XML servcies of output data
* Corrected types for a few of the binary attributes
* Updated docs for recent API changes
Resolved a few linting errors.