added SimpleBuildStep implementation for workflow-plugin support.

Hi,

I've added support for workflow-plugin (https://github.com/jenkinsci/workflow-plugin/blob/f6400d5fa39eef851fe0e483f9115e1b90c9be8a/basic-steps/CORE-STEPS.md#adding-support-from-plugins)

I just implemented SimpleBuildStep interface. Tests are ok.

Maybe you can merge it?

Regards,
Stefan