Add support for request ID in all services.

This is a breaking change that renames the `response.data.RequestId`
property to `response.requestId`. To migrate your code, change:

    svc.operation(params, function (err, data) {
      console.log('Request ID:', data.RequestId);
    });

To the following:

    svc.operation(params, function () {
      console.log('Request ID:', this.requestId);
    });

Closes #215